Undetected external montor on a notebook with Ubuntu 20.04

I have a notebook with ubuntu 20.04 and geforce gtx 969M laptop.
The external HDMI monitor is not detected (xrandr --listmonitors shows only the notebook’s display).
I’ve tried drivers 460 and 470 (both from repo and from nvidia’s runfile), and nouveau.
This seems to be a common problem judging from the posts on various forums,
but the solutions I found there are either not applicable or not working in my case, as far as I can see.
I would appreciate any help, I attach the nvidia bug report, generated with the external monitor plugged in.
Thanks. nvidia-bug-report.log.gz (759.8 KB)