Are you using the generic kernel or HWE (HardWare Enabled kernel)?
uname -a
You may need to install the mainline HWE or DRM kernel. Before trying that, I would suggest trying the /etc/gdm3/custom.conf edit above, anyway. If you have not tried that yet, I would still consider it a reasonable chance of working.
Otherwise, you can install the MainLine Kernels you need through Synaptic Package Manager to make it easier:
sudo apt install synaptic
Or
https://www.cyberciti.biz/faq/howto-upgrading-the-ubuntu-linux-kernel/
I am sorry about the links- I am majorly multi-tasking at the moment and trying to reply to posts quickly.