I have tried all of the troubleshootig options by none seem to work.
These steps and video seems to be the most helpful to fix this issue.
Doesn't really matter about the old phone as long as it is not within Bluetooth range and there is nothing else nearby within Bluetooth range.
Main thing is to remove the 45 from Garmin Connect app, "forget" the 45 in current iPhone's Bluetooth settings, then pair with Garmin Connect (plus sign, top right corner of the main page of Garmin Connect).
If that doesn't work, where is it getting hung up?
on iphone settings app
Pair only with the Garmin Connect App, do not use iPhone to search for or pair the forerunner. As long as your Forerunner is in pairing mode like this...
then go in to Garmin Connect app and add device - More > Garmin Devices > Add Device
At some point you should be prompted to enter the PIN being displayed on the watch.
Another video for specifically the Forerunner 45...