Ubuntu 3 monitor setup : monitor connected by usb-c or mini display port not found

I was having almost same issue. I found out that nvidia is driving only one external monitor at a time. Try disconnecting one of the monitor (HDMI-0) and see if the DP-2 one gets signal. It does not solve the issue, but if this is the case, at least you will know the root cause. See my linked questions for these: