A bit of an update:
The issue seems to come and go for the Android App and FireTV, while the browser app only works if I allow insecure connections (and sometimes that does not work).
It seems that the issue is fixed for now, the remote access seems to be working.
I am still seeing 422 (when I apply remote access settings) but maybe it is a red herring?
Oct 06, 2020 10:32:16.123 [0x7f70c57fa700] DEBUG - HTTP 422 response from POST https://plex.tv/servers.xml?auth_token=xxxxxxxxxxxxxxxxxxxx
Here are the mapping related logs:
Oct 06, 2020 08:50:51.466 [0x7f71017fa700] DEBUG - PublicAddressManager: Obtaining public address and mapping port.
Oct 06, 2020 08:51:05.338 [0x7f71017fa700] DEBUG - MyPlex: mapping state set to 'Mapped - Not Published'.
Oct 06, 2020 09:50:52.176 [0x7f70c7fff700] DEBUG - PublicAddressManager: Obtaining public address and mapping port.
Oct 06, 2020 09:51:05.383 [0x7f70c77fe700] DEBUG - MyPlex: mapping state set to 'Mapped - Not Published'.
Oct 06, 2020 10:19:07.322 [0x7f711b639700] DEBUG - MyPlex: reachability check - current mapping state: 'Mapped - Not Published'.
Oct 06, 2020 10:19:07.322 [0x7f711b639700] DEBUG - MyPlex: mapping state set to 'Mapped'.
Oct 06, 2020 10:32:09.886 [0x7f711b639700] DEBUG - MyPlex: reachability check - current mapping state: 'Mapped'.
Oct 06, 2020 10:32:09.886 [0x7f711b639700] DEBUG - MyPlex: mapping state set to 'Mapped'.
Oct 06, 2020 10:32:14.869 [0x7f70c57fa700] DEBUG - MyPlex: stop public ip check and mapping - current mapped state: 'Mapped'.
Oct 06, 2020 10:32:14.869 [0x7f70c57fa700] DEBUG - MyPlex: sendMapping resetting state - previous mapping state: 'Mapped'.
Oct 06, 2020 10:32:14.869 [0x7f70c57fa700] DEBUG - MyPlex: mapping state set to 'Unknown'.
Oct 06, 2020 10:32:16.124 [0x7f70c57fa700] DEBUG - MyPlex: mapping state set to 'Mapped - Publishing'.
Oct 06, 2020 10:32:16.124 [0x7f70c57fa700] DEBUG - MyPlex: mapping state set to 'Unknown'.
Oct 06, 2020 10:32:16.160 [0x7f711a637700] DEBUG - MyPlex: start public ip check and mapping - current mapped state: 'Unknown'.
Oct 06, 2020 10:32:16.160 [0x7f711a637700] DEBUG - MyPlex: mapping state set to 'Not Mapped'.
Oct 06, 2020 10:32:16.160 [0x7f711a637700] DEBUG - PublicAddressManager: Obtaining public address and mapping port.
Oct 06, 2020 10:32:29.980 [0x7f7101ffb700] DEBUG - MyPlex: mapping state set to 'Mapped - Not Published'.
Oct 06, 2020 10:32:29.980 [0x7f7101ffb700] DEBUG - MyPlex: sendMapping resetting state - previous mapping state: 'Mapped - Not Published'.
Oct 06, 2020 10:32:29.980 [0x7f7101ffb700] DEBUG - MyPlex: mapping state set to 'Unknown'.
Oct 06, 2020 10:32:31.281 [0x7f7101ffb700] DEBUG - MyPlex: mapping state set to 'Mapped - Publishing'.
Oct 06, 2020 10:32:33.576 [0x7f711b639700] DEBUG - MyPlex: reachability check - current mapping state: 'Mapped - Publishing'.
Oct 06, 2020 10:32:33.576 [0x7f711b639700] DEBUG - MyPlex: mapping state set to 'Mapped'.