Comment by maltris

For people who absolutely need to run 16.04, 18.04 and 20.04 without any further subscription, I recommend installing the signed kernel from 22.04 (as of right now only available in the -proposed repo). Worked fine for some 16.04 examplary machines I had around that are stuck and bumps the kernel to 5.15 (soon EOL, but maintained by ... canonical, to get security patches).