Fitness App

i recently tried to re-setup my partners Apple Watch which he hadn’t used for several years. However in the process the fitness app has frozen on one page and will not pair with the watch. I have an Apple Watch the same age which still works properly. I have removed the app and restored it, restarted the watch, restarted the phone. Nothing I do will get the app working again to pair with the watch. The phone is an IPhone XS and the watch is Series 2. Any suggestions how to resolve this please?