Hey there @HawkeyeWilikens!
If setting the resolution to 3840 x 1620 in the game's settings isn't working for you, have you tried editing the .ini file?
You should be able to find it by going to your Documents folder > Assassin's Creed: Valhalla > ACValhalla.ini, and the settings you're looking for are FullscreenWidth and FullscreenHeight. Adjusting those values accordingly should hopefully let you play at the desired resolution!