Refresh rate not working on login screen

Solution 1:

What about bypassing the login screen?

https://techpiezo.com/linux/enable-or-disable-automatic-login-in-ubuntu-20-04/

Alternatively, you could press Ctrl Alt F3 to drop out of the graphical screen to a text login.

I wonder if you get any useful information about your display manager with the following command: systemctl status display-manager

Sorry -- do you have a file /lib/systemd/system/gpu-manager.service?

cat /lib/systemd/system/gpu-manager.service
[Unit]
Description=Detect the available GPUs and deal with any system changes  
Before=display-manager.service
Before=oem-config.service

[Service]
Type=oneshot
ExecStart=/usr/bin/gpu-manager --log /var/log/gpu-manager.log
StandardOutput=null
StandardError=null

[Install]
WantedBy=display-manager.service
WantedBy=oem-config.service