Just to clarify, the AnomalyRO Settings app also uses OpenSetup (it opens it when you click Graphics Settings). The resolution override in our custom settings app is for specific cases where OpenSetup doesn't properly set the resolution / doesn't list the resolution you want.
@Rigonna89, you mentioned trying to run full screen at a lower resolution; unfortunately, this isn't actually possible - when full screen (or borderless mode) is enabled, it'll automatically set it to your screen resolution. The resolution settings currently only work for windowed mode. Right now the only way to play on a higher resolution screen is using windowed mode at up to 1920x1080. I did try looking for a way to override this in the past but I wasn't able to find a solution that actually worked; it seems pretty hardcoded in the client.
Try using the settings button in the AnomalyRO patcher. I've noticed this opens a different settings menu than the opensetup.exe located in the anomalyRO folder. This worked fine for me.
Should this not work, try the reverse. Open the opensetup.exe in your anomalyRO folder and select your resolution there. Ofcourse the above mentioned resolution limitations will still apply regardless.