There is a .ini file where you installed the game that you need to change plus another one in your documents folder. Both have the same info, but you have to change both for it to work.
Also any other .ini file you find take a look inside that it doesn’t have resolution stuff too. If you miss anyone it will revert back to its normal res. But I am pretty sure there’s only the two.
Also make sure fillscreen is set to 1