It should be possible to manually change the resolution in the config file.
Find this line "primary_adapter = '2560,1440,120'," and change the resolution so that it fits your screen.
Mind that the numbers here are from my own config file so they will vary from yours.
You can also find all the supported resolutions under the line "primary_adapter = {"
As to how get the config? Just open the client, click on the 3 stripes in the left top corner. This will open a menu in which you will choose the "show game prefs file". Open the game.pref in text editor and find the lines so that you can edit them.