That's why I used Google Maps in the few instances where I needed to re-route over a long distance. Ahead of the TCR I cached enough coverage for the entire race, so I always had quick navigation with or without a connection.
I didn't have to do any really big re-routes and had offline maps in the form of Google and OSMAnd. In the end I mostly used 1000 with RWGPS coming into plan after 'swimming' across Montenegro.
Then you require a data connection though.
Unless you have offline maps on your phone.