Your suggestion about remote access gave me a hint. I seem to have solved the problem by making two settings changes. To test, I tried movies and TV shows that would not previously play.
This is how I seem to have solved it.
In Plex settings, I changed the network type to IPV4 instead of both. I added a netmask for the LAN (192.168.12.*). I also changed the preferred network interface to be the physical. I unchecked remote access. I can’t do port forwarding from outside my LAN because T-Mobile uses CGNAT and my gateway is IPV6. I use Tailscale to get around this.
I recently took a trip to Florida and was able to play several movies through Plex without issue. I didn’t check to see if they were being transcoded, however. I expected Infuse to be able to play movies/shows on the same LAN.