A BR1 modem will get service where phones struggle. They will in most cases keep RTK on MNCORS in fields where cell phones struggle to send text messages. 

I have been using MNCORS since 2015, and lines I made then are still repeatable. I have not seen issues of lines shifting, the explanation you mention sounds questionable. Corrections come from the MNCORS server via the internet, not the cell towers. As long as you access the internet you are good to go.

I setup all my MN guys with MNCORS and everyone is happy with it. I have a pile of them running all over the state. It makes economic sense for the farmer vs a subscription. TerraStar is a great product and my non-MN guys get good service from it. 

You can get a demo RTK unlock to try out on your 7500. A MNCORS account is free. If you can get a BR1 and cabling loaner from your dealer of choice you could try out MNCORS for free.