how to never lag?

it is not on the list

I do have problems in other games and internet itself sometimes but they are impossible to predict or fix, sometimes it works completely fine, sometimes it lags for no reason whatsoever, but there is nothing I can do about them because when it lags, nothing exists to indicate why, nothing else is using the internet and restarting modem/connections does absolutely nothing for longer than 10 seconds if at all, it also lags not just on this one pc but on anything connected to internet through modem, the constant calling of isp to fix it just works once if it does at all, then goes back to trash again after a while

wire is kind of not an option because it would need to be like 20 meters long and go through multiple walls, idk if I can even get a cable long enough

also if it was a problem between pc and router, how can it sometimes work completely fine? the geometry of the place never changes to have a different effect on the connection between the 2, idk any tests for checking packet loss between specifically pc and router either, they all seem to only exist from pc all the way to destination which doesn't tell a lot about where the problem is, only that it exists at any point between, the one I usually use is https://packetlosstest.com/, and often have packet loss to UK server but not to others, atm for example, there is zero packet loss to even UK server, but that can change easier than weather and even less predictably

I forgot to ask, but does vpn have any effect for anyone who had similar problems?

Do you use a free vpn?
Do you have a virus or cryptominer?
When issues occur, can you surf the internet normally?
Is anyone else using your router?
Have you changed the default password on your router?
Could someone else be using your wifi?
When issues occur, what are your results when you use a website that tests your internet speed?

no vpn (that's why I asked if it would help to use one)
no virus or crypto
internet is normal even then for browsers and other stuff
yes but so are they when it doesn't lag
no
yes but so are they when it doesn't lag
same as when there is no lag (~20mbs download, 10 or less mbs upload), on packet loss test one though it can have some losses

EDIT: another thing I notice is that the lag is usually specifically towards people from ireland, rarely to anyone else, weirdly, and the country isn't even far from here while having no lag to countries far further away

You are using wifi through multiple walls and over a significant distance. I'm not surprised that the wifi can sometimes experience packet loss. I really urge you to at least temporarily try out a cable to compare the situation.
I got a 20m flat ethernet cable for my home. It's so flat that I can even fit it through doors without issue. That fixed all my internet issues and your situation seems pretty similar.
At this distance the signal strengths is weakened and basically anything can disturb the transmission. The neighbor using his wifi, the neighbor starting his microwave, Basically anything else that causes electromagnetic waves. I'd say it's practically impossible to detect these things.

long ago i have heared advice to disable extra network stuff (see screenshot) as people claimed that ice adapter would search all your network stuff and having to much of it may confuse it or slow it down. Can't confirm that it helps.

Also you can try to run ping or better something more advanced (no idea what, but something that would sent packets more than just once a sec like ping does) to check if your connection is ok, you can simulteniosly run this and check connection to router and to something in your country and to something in another county and maybe when you have connection issues you will see lost packets everywhere or you will see packets go to router but fail to go anywhere else, or you will see packets go to router and go to some server in your country but fail to go outside your county or something like that, can set more those checks just so you can know if that is something with your internet, or with your provider or with some magistral(?) - providers of higher tier that sell bandwith to regular providers

ok I tried disabling all other network devices, will have to wait and see if there is any noticeable effect

If you are using Wi-Fi as you cannot run a lan cable for any reason, get a TP-Link AV1000 Powerline Starter Kit to send the internet through the electrical cabling in your house. I'm pretty sure it's witchcraft but it does work!

^Ethernet through powerline is a good option if you can't run a dedicated cable. Linus Tech Tips on youtube actually just put out a video about it yesterday I believe.

@Mach
As BlackYps said, cable length isn't a problem unless it's like 50m+. Also, what you want to look for, as Yps already mentioned, is a 'flat cable'. They are made so that you can have them go in between doors and door frames. So there is no drilling or other destructive work needed to be done - you might want to get some wall-mounted cable holders to keep things tidy. The flat cable should work as long as you don't need it to pass trough PVC or anti-burglary doors (which I'm assuming you don't, those are usually installed for doors that lead to the outside, not in between rooms), or you have some super-ultra air-tight door, which is rare for indoor doors.
Regardless of your ability to get such a cable, I'd strongly urge you to do as Yps said and at least get any kind of cable, and move your setup temporarily to another room to check if connecting via a cable really fixes it - if you conclude that it does, then you can ponder on how to actually get he cable into your room.

Also, if you look around, you probably can find a PC shop in your area that can get you any-length cable made (as long as it is less than say 100m).

the cable part is unfortunately non-negotiable, the problem is that it would be an "eye sore" to have it running through several rooms, no matter the drilling of walls being needed or not (which in itself separately is also out of the question), nor the cable's appearance unless it is literally invisible

I may try the powerline adapter solution though since it seems it would have same effect as a cable without cable's problems, but I need to check first if disabling those devices changes anything as well, only way to check so far is to try playing like this until I either run into same problems or it just works fine on its own (or more like waiting until I run into same problems, to prove it's not that)

and once again I forgot to mention something, but when there is a problem to specific player, when joining a lobby without that specific player, there is no lag ingame once again, without any changes being done to anything, which makes it look as if the problem is outside this entire area in first place and invariant of its interior connectivity (since it can be lag free even with wireless just minutes after it was lagging, just by there not being the specific player in game that the lag was solely towards), and is also why I don't understand how everyone sees me lagging but not the other player since problem obviously only exists between the 2 of us, like I have to remind people "look how I am not lagging now when this player isn't in game" and they will still think it was solely my problem and dismiss the current evidence of persistent no lag situation