Sorry to hear about the issues but his is a bit of a weird case. I wonder if the weak signal was worse than no signal at all. Map Pilot is able to save planned missions in Airplane mode (no connectivity). After a few warnings that things are not being downloaded the mission is saved and is available in the Mission Manager list. Did you check there? In this situation you might have to just give up on downloading the background basemap data and switch the data connection all the way off in order to save the planned mission.