Q. How did John get the letters he wrote to the seven churches?
A. Perhaps he had periodic visitors or found someone he could trust from the supply ships to carry the manuscript to the mainland to be copied and distributed.
Or, since the Revelation was written so close to the time he was released from Patmos, he could have carried it himself and arranged for its distribution personally.
He wrote the book in 95 AD, then was released and spent a few years in Ephesus before dying of natural causes in 98-100 AD.