I don’t run plex myself inside a VM so I’m sure others more knowledgeable than I may jump in but this jumps out in your logs…
Jan 25, 2022 20:31:23.721 [0x7f5dd7946b38] ERROR - MyPlex: mapping failed due to potential double NAT configuration
Jan 25, 2022 20:31:23.721 [0x7f5dd7946b38] DEBUG - MyPlex: mapping state set to 'Mapped - Not Published (Double NAT)'.
There are also other network related errors in your logs so I would suggest fixing that.
Edit: In this doc Synology FAQ - Questions, Answers, and a few How-To's - #35 by ChuckPa I see the following…
- Set your network. You want to use the Host network - NOT bridged - NOT NAT .
