Mnemonic vs IP

The IOS app works really well for me, BUT only if I use the IP address of my HA server. However, if I put my mnemonic (DDNS) address in, I get “Could not connect to the server” error.

DDNS is configured fine, as is port forward: If I put my mnemonic (including the port number) into the web browser, it works fine.

Any ideas?

Are you testing this with your iOS device on wifi or just on cellular?

Doesn’t matter if I use my home WiFi or use my phone in 4G mode with the WiFi off. Same result. That’s the bizarre bit. I can’t even claim the router’s just doing its job and routing!!