Plex server over mobile broadband not working

I have a working plex server that allows me to watch my recorded TV programs when I am away from home (UK). I have access to my neighbours ADSL but the speed is slow.
I now have a new 3G mobile broadband connection which is 5 times faster on speed tests.
It does not work with Plex on any if my devices (ROKU, Android phone or Android tablet) or with Plex web app on Chrome. I get messages which say datarate too slow (but same recording works fine on ADSL connection) . I can access the server (slowly), see the thumbnails (slowly) but the recorded program will not play.
If I remote access server using TeamViewer I can play recorded program without problems and at fast speed.
So my connection speed is good, the recordings are good but there is an issue between mobile broadband and plex.
Does anyone know what it might be ?
Thanks

Some mobile ISP’s do not allow you to connect directly into your server. Are you sure you are actually connected to the server and not just showing cached data?

Please restart PMS then provide the logs while running on the 3G network.

More like mobile network use mostly NAT and you monstly never get a dedicate IP

With the increases use of CG-NAT also make this more difficult,

The only way to do it would be use a VPN with preferably a static IP (and no nat of course)