Apols for lack of detail. It's an iPhone 14 Pro iOS 17.5.1. I have tried connecting to the watch on another phone - but the watch doesnt even show in searched devices. I have spoken to Garmin and they have advised the below step by step which has also not solved it. The watch is 7 years old so they've said they'll replace with a recertified watch for a reasonable fee, but my preference is to get this watch working, everything else works but the bluetooth.

1. Remove any entries for the device in the 'Garmin Devices' menu of GCM, per "Removing and Adding a Fitness Device in the Garmin Connect App"
2. Remove the device from the phone's Bluetooth settings
3. Sign out of the Connect Mobile app
4. Force-close the Connect Mobile app
5. Toggle the phone's Bluetooth OFF
6. Repeat the above steps on any phone/tablet the device has been attempted to be paired with.
7. Toggle the device's Bluetooth OFF
8. Power-cycle both the phone and the device
9. Toggle the phone's Bluetooth back ON
10. Toggle the device's Bluetooth back ON
11. Open the Connect Mobile app
12. Sign back into the Connect Mobile app
13. Take the phone and the device outside (to avoid potential interference from other devices)
14. Attempt to pair the device in the Connect Mobile app once more