Well done Dro,
Early versions of PilotAware were designed to take GPS position from tablets or other external sources, however this proved to be unreliable in practice, so the system was changed a considerable time ago to require a dedicated GPS source. To prevent the possibility of transmitting false data, the system is set up so that it won't pass data to the transmitter or Nav system unless and until it gets a positive GPS fix, hence it won't work without a fully functioning dedicated GPS source. Unfortunately, the problem wasn't obvious from the information you initially provided. I'm glad to hear that with Lee's help you have now finally got to the root of the problem.
Best Regards
Peter