Oops. For trailer TPMS we elected NOT to use the Ford solution. Big reason was we like to monitor tire temp/pressure trends when driving and did not like how Ford dash handled that. We manage our trailer camera and trailer TPMS via the Haloview BT7 system. My simpler to use and works best for us.

 

When I installed my factory supplied TPMS kit the tire sensors were detected and I was prompted to program the sensors when I hooked the trailer up. I don't have my trailer handy to check but I believe that the relearning is on the steering wheel menu button under Towing/Trailer tire info.

EDIT: If the above doesn't work go into the towing menu on the center display and after the trailer is hooked up and select new trailer.