Ah ok, you weren’t getting dropouts on 8.13 before you upgraded to 8.20 - but you also weren’t using the WireGuard VPN protocol yet which was causing them. 
That’s a better scenario for you! Get WireGuard working on 8.13 when you have time, and then hope 8.20 will work.
I have never seen a modem show up as “no device detected” until downgrading back to 8.13 from 8.20 . So… that’s exciting. I’m hoping that the hard locks won’t happen anymore. Time will tell.