Ubuntu 20.4.6 LTS Nvidia legacy 340 driver and external monitor

[Richmond]

Hi,

I installed Ubuntu 20.4.6 LTS and the Nvidia legacy 340 driver. I
configured the external monitor using nvidia-settings, and wrote the
configuration to /etc/X11/xorg.conf. So far so good, it boots the system
and displays the nvidia banner and the external monitor is working, but
then when gdm3 starts up the graphics card is reset to its default
settings, which are not quite right. I tried to change these settings in
the gnome preferences - displays, but it doesn't quite work the same and
sets the virtual screen bigger than the screen. I can work around this
by using xdm as the display manager and ctwm as the window manager. But
I would like if possible to get it working with gnome. So the question is:

How do I stop gdm3/gnome resetting the graphics card?

Chatgpt suggests this is related to composite windowing or some such
thing. I installed gnome tweeks but I can't find any settings for window
manager in the preferences or in the tweaks. I cannot see any option to
choose between wayland or xorg, this should be on the login screen
should it not?

I set kernel parameter nomodset and removed quiet splash.


Inspiron-1720 5.4.0-193-generic #213-Ubuntu SMP