This has been referred by @ChuckPa to the Plex for Android development team
I would like to eliminate one area
On a PC on your local network (192.168.100.x subnet), could you open a command line window and enter this command
nslookup 192-168-100-4.d889772e5ef141a1a08ec934305c2211.plex.direct
and copy and paste the response you get back here
Just want to check out there is no DNS Binding Protection angle on the problem
What is the make and model of the router?