Revert option - Manual install of 18.04 (if you’re on FP open, different images are needed, but as you don’t mention that you are, I’m assuming you’re not):
Fix option - Not sure what happened, but related to that I’m also interested in the answer to the question @Amber just asked.
I’ve been having the same issue since the update. With two SIMs, neither of the two gets recognised. By swapping them before booting, I can sometimes get both to work simultaneously, but as soon as I reboot, I lose them again. If I take out my second SIM, it recognises the first SIM every time. For now, that seems to be the only stable way to use my phone, but it’s of course not really a satisfying solution.