If you believe hard enough that the government is trying to screw you, you could think that our taught satellite physics is a parallel construction https://en.m.wikipedia.org/wiki/Parallel_construction and the real thing actually conforms to flat earth... with enough epicycles.
Your phone could do arbitrary 'magic' (ie technology) to get good positioning.
You would first need to make some kind of pretty complicated argument why your phone couldn't do this without satellites.
Somewhat easier for you, and harder for flat-earthers to push aside: the ISS is just about visible to the naked eye and definitely to someone with a backyard telescope. Similar for Starlink satellites.
In my experience, flat earthers are not capable of the kind of rigorous enquiry that would allow them to even attempt to answer this.
Flat earth isn't a position arrived at by reason, it's one that is invariably reached by either confusion, or as a necessary consequence of some unshakable core belief (typically either an extremely literal reading of the bible, or a paranoid "everything official is a lie" delusion).
