I was able to pair my new Forerunner 945, but I kept the 735XT on garmin connect too. The only one that works 1 week after is the 735XT. I can't find the Forerunner 945 again with any mobile
Remove your 945 from the Garmin Connect App, if it exists (Menu >> Garmin devices).
Unpair the 945 in android Bluetooth settings. Settings >> Bluetooth >> Unpair
If the 945 is paired with any other nearby mobile device, remove it there as well. Garmin devices work best when only paired with a single mobile device
Restart your phone and the 945
On 945 place into "pairing mode." Differs per device, on newer devices: Menu >> Connectivity >> Phone >> Pair Phone to manually enter pairing mode.