I didn’t have great service in large patches of the route, but the gps works regardless, so as long as you have the GPX files, it should point you in the right direction.
it’d save a lot of battery when airplane mode is turned on in those areas as the phone doesn’t look for reception constantly


i use öffi for public transportation, maybe there’s something similar for your region? or maybe your region is already included, check here: https://oeffi.schildbach.de/