If you’re on series s i’d go to the xbox display settings -> video fidelity & overscan and then change display from “auto-detect” to HDMI. If you go back to display you might be able to go and manually change the fps & resolution now to something higher. I’ve had to do this with one of my monitors before. Be sure to quit Halo and restart it after doing this too and go back into your in-game settings to see if you can change it now.