Does anyone know how I can use a lower resolution and FPS bug online

Does anyone know how I can use a lower resolution but keep the screen full? Whenever I try to go full screen, it simply reverts to my native resolution. I believe that if they optimized it more, it would be fine to use the native 2K 3440 x 1440 resolution. I'm wanting to use 2K 2560 x 1440 16:9 because in online daily mode my FPS drops a lot. The last session I joined I had 30 FPS and had to quit, which is strange because if I join an online server, even a full one (from the server list), the FPS is still almost the same as playing offline, around 80/70 in 2K 3440 x 1440 . Does anyone know what to do about these two issues?