I have Infinity (no Virgin up my street

) and have a VPN Server (Synology) so that I can access my home network from work. I want to be able to test this easily so have BT-FON enable and connected my iPhone wifi to the Public side of my WiFi network using my DYNDNS type domain name.
I know that port-forwarding works as I can access the network from work/mobile data network but if I am on my BT-FON connection I cannot access my Network via the resolved DNS name.
Anyone know if this is fact and it isn't possible? Unfortunately the HH3 is so locked down that I cannot setup and debug type commands ...