just use cable! as my understanding is SUpCom doesnt need super fast internet, it needs super stable internet. Otherwise you fall behind in data. a 300 ms ping isnt that bad if your con stable and better than a 40 ms unstable con. (most times high ping is indeed an indicator for unstable con, but it dont have to) I had the same problem, 1v1 worked okay the rest was lagfest. faf works peer to peer not over server, so minor unstabilitys which you wont recognize in other games mean you are 1 to 3 behind in data in faf.
cable is always better than wifi, you can have the best wifi router and your pc 30 cm away from the router, a 1,95 $ ethernet cable will still beat your 500 $ wifi setup.
The game checks every tick every player and the game does not proceed till everybody got checked. Its different to shooter-like games, there the server assumes when you move in one direction, that will do also while you lag a little and you wont realize. i dindt understand this in the beginning, my wifi works fine for shooter, but i was always lagging in sup com (sorry to everybody who played with me 7 months ago) Now i got cable and everything is fine.
also sup com calculates a lot more than other rts, projectiles are traveling trough space and time in real time, ballistics have to be calculated, may there be an asf in the way of an arty shell? is the nuke getting cock blocked by sattelite? it seems a little over enginiered sometimes, but its what makes this game so big. just try it out with cable at least.
as i said this is my understanding, it may be wrong, but cable solved all my problems.