Can conect to https://x.duckdns.org:8123 but can when local

Configuration
1

This is verry odd. I can connect using my duckdns.org subdomain from my laptop and mobile (using WiFi) but only if my mobile is conected to my local network (if I turn off WiFi and use mobile data it will not connect).

Recently I canged my router. I have the router I was given my my ISP (British Telecom) and a TP Link one conected to it (the one I connect my mobile and laptop to).

I have a port forwarding set up for both for 8123. This is what I previously did with my previous simeler setup with the suplied router and my old Rock Space one. So it works if I conect to the BT or Link one wit the full URL but not if I try to conect using mobile data (mobile data is working fine for everything else).

The Pi With Home Assistent is conected to the Suplied BT router with a wired conection.

Just to reiterate the same port forwarding worked fine with my previous router.

2

Sounds like the new router does not support nat loopback aka hairpinning aka reflection.

By default many routers only port forward for requests coming from the internet and will not redirect requests made to the external interface from the LAN back to a LAN host.

Make sure there are no options to enable it 0n your router.

3

Aparently the router suports NAT loopback by default (AX73 AX5400 Loopback - Home Network Community). Would Port Triggering or puting the Pi in the DMZ maybe a way to go?