Mac Mini monitor defaults to 720p and 60Hz after unplugging HDMI merger

My partner got a new Mac Mini about a couple weeks ago and has been using a 1080p 100Hz monitor for the entire time. The HDMI is routed through a 4 in 1 HDMI merger which hasn't caused us any troubles until now. I accidentally unplugged the merger, and when plugged back in the monitor now defaults to 720p and wont go any higher than 60Hz. Is there any way to fix this?

[Re-Titled By Moderator]