LinuxCNC on Linux Mint 20 quick and nearly easy install

Hello, my name is Rudolf and I installed everything as written on the first side of this thread, but whenever I start with the "Linux Mint 20 Xfce, with Linux 4.9.0-13-rt-amd64" Kernel the display resolution is 800x600.
I cannot mend this, has anyone an idea to solve this problem?

Further Infos:
The RT-Kernel and LCNC are working fine.
Motherboard: J4005B-ITX, graphic card onboard
CPU: Intel Celeron J4005, 2x 2.00GHz, 4MB Cache, 10W TDP

What does
xrandr
in a terminal say?

tommylight wrote: What does
xrandr
in a terminal say?

Okay, I opened terminal and "xrandr" says following:

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 800 x 600, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
800x600 75.00*

Try this: Reboot. See if the video works.

... yes now it works!
[strike]But whenever I restart, the old display resolution 800x600 again is present.[/strike]

...no it works, there is a new kernel (5.4.0-0.bpo.4-rt) now isnt it? With the new kernel it works....

So far so good. I haven't tried to run my mill, but all the software is in place. I'm using the same machine. I saved the configuration files before the install. If I put them back in my home directory will the new install pick up the old settings?

Also should I do updates in the LinuxCNC kernel?