FYI, here’s the manual.
So, the TuneECU app must be registered but the bike also has to be registered. Are you getting any % completed during the download?
You can use the ObdLink app to update the firmware in the MX+.
Regarding the battery, I have seen comments that if the battery is on a smart charger, it may appear completely charged but the power may drop when you tax the ECU. Have you tried a car battery? I used Tuneecu about 4 times to download maps to my T120 and never got a clean download. Fortunately, it always recovered.