Using this amazing project tmate.io you can SSH tunnel to your colab notebook machine. You can with few lines of code download the binary, run an instance in the background and output the generated SSH address.
!wget -nc https://github.com/tmate-io/tmate/releases/download/2.4.0/tmate-2.4.0-static-linux-i386.tar.xz &> /dev/null
!tar --skip-old-files -xvf tmate-2.4.0-static-linux-i386.tar.xz &> /dev/null
!rm -f nohup.out; bash -ic 'nohup ./tmate-2.4.0-static-linux-i386/tmate -S /tmp/tmate.sock new-session -d & disown -a' >/dev/null 2>&1
!./tmate-2.4.0-static-linux-i386/tmate -S /tmp/tmate.sock wait tmate-ready
!./tmate-2.4.0-static-linux-i386/tmate -S /tmp/tmate.sock display -p "Connect with SSH address: #{tmate_ssh}"
!./tmate-2.4.0-static-linux-i386/tmate -S /tmp/tmate.sock display -p "Connect with web: #{tmate_web}"
When I first used it, I was able to be connected for about 15 min. Now, not even 15 seconds. I get runtime disconnected.