Try installing your motherboard's AMD Driver for your APU and see if that makes any difference: https://www.asus.com/Motherboards-Components/Motherboards/TUF-Gaming/TUF-GAMING-B550-PLUS/HelpDesk_D...
When you connect both Monitors what does:
1- Windows Settings - Display show? It is possible that you have only one Monitor as your Main Display monitor unless your Extend or Duplicate your second Monitor.
You might need to configure both to work similar to what I have when I have my DP Monitor and HDMI Television connected on my GPU Card.
Display-1 is my PC Monitor and Display-2 is my Television set. Display-3 is not a monitor but my A/V Receiver connected via HDMI cable.
2-Device Manager - Monitor and Display show or any errors?
Under Monitor does it show 2 drivers? one for each monitor or just one?
3- Run DXDIAG.exe and save the file and see if both Monitors are showing or any errors?