This is most likely where it says a double NAT situation, this is rarely and internal network issue it more than likely that your ISP issue you with an IPV4 address but as these are now limited (run out of them) ISP’s are using NAT to direct traffic to your home router, As far as the internet is concerned though you are using the same IP address as many others on the same network, so as Plex updates your IP address to its cloud to allow remote access it only sees the shared IP address, so when it tries to connect it cant find your actual IP address as that is hidden on the internal side of the ISP network.
Think of a network within a network
Your device connects to your router with and internal address 192.168.*********
Your router connects to the ISP and gets an IP address on their internal network 92.38.******
Your ISP connects to the internet with its own IP address 102.89.*******
Plex tries to connect to 102.89 address on port 32400 or whatever one you use but cant find your server as its IP address is not actually 102.89******* it the 92.38 address which might also be dynamic (meaning it can change each tiem your router re-syncs with you ISP).
Year ago when there were plenty of IPV4 address this was not and issue as the chances are your router had an internet facing IPV4 ip address that may stay the same for ages or change but it didnt really matter as it was directly connected to the internet.
No ISP use CGNAT carrier grade network address translation, which means it has rule on it own network to rout traffic form its own external IPV4 address to you internal IPV4 address on its own network.
This is the most likely issue for remote access not work aside from you own internal firewall possibly blocking it.
If you look up whats my IP you will probably have and IPV6 address (which is fixed and stupid long) you can then add this into the custom URL section on the Settings page under network
It should look like this https://2a0e:cb4:13:d300:107:4b867:9622:67c7.******.plex.direct:32400 just find your IPV address and paste it inbetween // here and .plex.direct:32400 (the 32400 is the default port plex uses)
You then need to go to the top of the page and tick the enable IPV6 support
I use this method and it does work but can take a few attempts to connect up as I think it use the IPV address it sync first before trying the custom URLS.
Hope this helps
Its all in the help command on the Plex forums and stuff just serach CGNAT Plex or similar
