Edit: I dont think Smartphone link works on 3790, and IMHO cant be enabled .
it's the phone sends a pin to your nuvi when you pair it, you must just accept the pairing.
Try to delete the pairing, restart your Nuvi and repair it again from phone(search for new devices in BT settings).
You wouldnt loose your LT converting to 34xx, tested by me and many users here on Noeman