The dedicated iFly devices are notorious for having trouble consistently reconnecting to wifi. This problem mostly seems to occur when the login credentials are saved/remembered for more than one network. Even if both networks are never in view at the same time, the device still "gets confused". The best way to avoid this issue is to only save the login credentials for your ADSB network, and if at all possible, just never log into any other wifi networks. Otherwise, you'll constantly be battling this gremlin.
I don't have a 740 so can't give you the exact steps to delete/forget the saved credentials for a network, but I think they show up in the list with a "(Pre)" in the name. If the "(Pre)" network(s) is(are) not currently in range, they may only show up briefly right after the device is turned on, so you might need to reboot a few times to hunt them down and remove them.
Someone else who has gone through this with their own 740 might be able to give you better/more specific instructions.