Is that Russian ship still parked nearby? I recall reading about that a while back. Maybe that's where the signal is coming from. https://www.cnn.com/2018/01/22/politics/russia-spy-ship-us-c...
https://en.wikipedia.org/wiki/Timing_advance
(Note there's and "extended range" feature, where you can halve the cell site's capacity by waiting two timeslots in the TDMA schedule instead of one - which lets you go as far as 120km...)