I think Adventure Pilot would love to know the answer to that question. The root cause of the 720 and 740 devices' wifi flakiness has been a mystery to everyone--AP and users both--since day one. Unfortunately, the wifi support is baked into the device's core OS+driver software as it is delivered to AP, and is thus outside the scope of the iFly software that AP writes and controls.
As someone who used a 720 for years and dealt with wonky wifi the entire time, I feel your pain and share your frustration. But unfortunately, you're not going to get an answer to your question because no one who cares (at AP or in the iFly user community) knows, and no one who might possibly know (the original hardware platform vendor) cares. The hardware product is some 7 years old now and no longer supported (and the hardware platform vendor support to AP was not so robust even when the product was new).
For what it's worth, you question why it worked for a while and then stopped. From my perspective, I am amazed to hear that it ever worked for you in the first place, since mine never did....