Hopefully, someone can help me out with this as I'm slightly stumped.

NZB 360 Android v12.3.1 (Pro)
Sonarr Version, Mono Version
Radarr Version Version, Mono Version5.12.0.301 (2018-02/4fe3280bba1 Fri Jul 20 08:25:42 EDT 2018)
Lidarr Version, Mono Version
SABNzbd v2.3.5
TP-Link Archer 3200 Router Firmware Version:0.9.1 0.1 v004b.0 Build 160712 Rel.55313n

From NZB360 on Android I can connect to SABnzbd, Sonarr without issue over GSM/4G/WAN and of course internal network

If I, however, try to connect to Radarr or Lidarr they both fail with "Bad Request (Invalid host)" for both applications. However, over the internal network, all is well.

Firmware on the router hasn't been changed. I have NAT forwarding setup for all four apps including Plex and all but Radarr and Lidarr work without issue. I've removed the forwarding reloaded the router configuration, altered external ports but nothing appears to work now. This started a few weeks ago so not sure if it is a Radarr and Lidarr issue or otherwise.

Please tell me I'm not going mad.

Cheers in advance.