How is the monitor connected? Try changing the connection method. I suspect you are using an HDMI connection. Have you tried disconnecting and reconnecting the cable?
I have an LG Ultra HD monitor and and have no Overscan slider on either my Studio Max connected via the Studio's native HDMI port or from my 2013 Mac Pro connected via mini displayport>HDMI adapter. For both I use an HDMI input on the monitor.
Are you experiencing problem with the desktop ether filling too much of the screen or there are black barsnoround the desktop?
It appears that the slider was eliminated some time ago but you may be able to control it via code. See near the top of the second page of:
Underscan slider missing in Display set-u… - Apple Community
You can also see:
https://origin-discussions-us.apple.com/thread/253364474
Issue about overscan setting on Monterey