If that one mile happens to put you on a Navy base, that may be the problem. I don't know if T-Mobile has ever been able to put a tower on the Naval facilities in San Diego. Service in Coronado is not very good. There's only one tower there. I don't think they have band-12 fully deployed in that market at this time.
We want you to have the best coverage available. I can’t see your exact area to be able to determine if there is an outage or anything but I would recommend that you Contact Us so that our tech support team can have a closer look.