Thanks to a very kind poster in this MacRumours forum, you can get the three month’s free Fitness+ trial (rather than the one month offered despite having bought a new Apple Watch after September 15th) simply by unpairing your Apple Watch after upgrading to iOS 14.3 and WatchOS 7.2 and re-pairing them again. Then go to the Fitness app on the iPhone and click the Fitness+ tab – you’ll then get the 3-month offer.
This does mean that you’ll have to do some setting up of the Watch again, but I tried this myself after spending over an hour on the phone to Apple Support trying to fix this thing and getting nowhere with them. Thankfully it worked, and I’m now going to put it through its paces over the next few months because I bloody need to get into shape.
The whole experience of trying to get this trial was incredibly frustrating. Apple demanded I give them proof of purchase despite this being a Series 6 watch which only launched after September 15th. It was activated by me on the 17th September and I bought AppleCare+ for it on the 19th. Besides which, this is a Watch provided by a carrier so I don’t have proof of purchase as such – but EE weren’t helping by not updating the plan page with the correct device – they seem to think I still have an Apple Watch Series 4.
Honestly, Apple’s quality control is slipping again.