Thank you Squid. I've seen this reply on many threads with port questions. I thought it only applied when in host mode. The Plex docker does have its own IP on the proxynet subnet. 172.18.0.4.
As a general question though, why am I not able to add a new port mapping to a container? The config page let's me create and apply, but it doesn't actually apply?
Maybe I am barking up the wrong tree for why Webtools is inaccessible, and I will post in the Plex container forum.