My fingerprint reader stops working occasionally too. I've found that giving the power button a short press to lock the phone and again to unlock it keeps me on the same screen and gets the fingerprint scanner working again. It happens less since the last Verizon update.
Randomly restarting I haven't had and can't think of anything that would cause it. I would think, if you called support, they'd say to do a factory reset. I hate doing that but they do work occasionally. What to do depends on how much you like the phone. I like it, especially for the earphone jack & sound, so I would try the factory reset as a last resort. I just read that LG is releasing a bunch of new phones within the first quarter of 2021. They're doing different things with their screens, if you can wait. Good luck.
BTW: I'm not big on used phones, even refurbished. I've never had much luck with them and seem to inherit other's problems.