Thank you for your answer. Sadly I already updated to Kernel 5.8.0-16 knowing the 5.4 is too old.
On Ubuntu 20.04 a kernel update combined with your fix from nvidia-xconfig doesnt do what i want it to, nor does nvidia-settings - #7 by generix works like a charm. But sadly not on Ubuntu 18.04
You are probably right, the problem is the amdgpu and not the nvidia one because with “primaryGPU” “Yes” I got the external displays running. But the internal stays off.