When I looked in the manual of the AOC 24G2U, I found out it DOES support 144, but only on HDMI and Display Port.
You ruled out a defective DP cable., but have you also tried HDMI?
If it still is 60 on HDMI, it might be another problem or the monitor is not working.
A too low PSU can give problems with highend GPU's but the problems are more like crashing and hanging in games, Complete freezes, triggering powerbreakers/fuses, artifacts, not showing a screen at all, etc.
I have not heard or seen a failsystem that blocks the refresh option when a PSU is too low. Problems would be more severe.
Looks like something else.