Having exactly this issue as well - any help greatly appreciated
I’ve managed to resolve my issue. I was trying to pair the 520 with my phone through the iPhone settings and then trying to pair through GC. It doesn’t want to do that. Forget the 520 in phones Bluetooth settings, forget the Phone in the 520 phone settings. Then reconnect but only in the GC app not in the phones Bluetooth settings. I’m not that confident that this will last and i expect to have to do this again in a weeks time but the source of my problem was forgetting that I should only pair the phone via the GC App.
After upgrading to latest Garmin Connect and a IOS upgrade my 520+ would no longer upload rides. Like many others i tried all sorts of fixes?? it now works after I deleted as far as possible all refs to the 520+ on the phone, I then created a new name for the 520+ on the device before pairing. I removed the connect app from the phone and downloaded from istore switched off and restarted the phone. Signed in to the connect phone app and found the 520+ on the list of devices and started the pair process on the connect app. Then set the 520+ pairing. phone found my "new" device and sent it a code to prove its identity. then it all worked. what a nightmare!!! I am not sure which bit solved the problem by hey it worked.
well like many others, the edge 520 plus fails to sync with the garmin connect app. like others, i've tried different techniques and then magically it would start working again. and then it wouldn't. so this time i got a bit more serious and also logged my steps. it worked on two android phones. here are the steps
on the edge 520+, rename the friendly name. a bit tedious since you need to use the up/down keys to scroll through the alphabet. so my new device was named edge - a bit easier since all the letters are close in the alphabet. to save, you need to scroll above the A letter to the check mark. and i'm not sure if this is really needed, but it was a step i always did. saw it mentioned in one of the prior posts.
then on the 520+ - under menu/settings/phone deleted the connected phone and restarted
on phone – delete app, turn off bluetooth, reboot
reinstall app, turn bluetooth on
start app, sign in, add device - had to browse through the list of garmin devices to select edge 520 plus, then go through setup questions
you data will be repopulated from the garmin cloud so you don't need to worry about deleting the app.
hopefully, i'll be able to connect and sync for at least a couple of months. and when i can't i'll repeat these steps again.
also saw this post https://forums.garmin.com/sports-fitness/cycling/f/edge-520-520-plus/310364/edge-520-plus-bluetooth-connect-issues
seems like the answer might be the new app version 4.60