Switched SIM to old phone and back to newer one, now cell data isn't working.
Yesterday I took my iPhone 8 Plus in for a battery replacement. Since I had to leave my phone and pick it up later, I took my SIM card out and put it in my old iPhone 5s. When I did this, I got a notification on the 5s saying "our network is being upgraded and the cell phone you tried to activate will no longer work". I got my 8 Plus back last night and switched the SIM back but now I can't get the cellular data to work on it. What do I need to do to get it activated again?