Try disabling CSM (Compatibility support module) in the BIOS.
If that doesn't work, you can try doing a hard reset of the BIOS. If you changed monitors since the last time the BIOS was updated or reset, that could be part or all of the problem. Usually, it's not, but it has been known to happen with the UEFI not showing full HD resolutions. Also, in some cases, if a board comes with a limited BIOS ROM size and you update the BIOS at some point late in it's life, it's possible that the manufacturer may have had a need to include instructions and microcode that are large enough to require reducing support for HD display, such as with the recent MSI motherboards that were updated for Zen2 support.