Hi All, I have done what was recommended by the member "cconst" in the below thread and it worked for me with my Xiaomi Mi A1 and Vivosmart 4.
The key was to do the "Reset Wi-Fi, mobile & Bluetooth" step but here are all the steps I took.
1. Forget Garmin device from phone's paired Bluetooth devices
2. With a fresh install of Garmin Connect, sign in and initiate device setup (should be automatic anyway)
3. On the Garmin Device, ensure Bluetooth is on and hit the pair phone button (on Vivosmart 4 it is the Sync logo above the Bluetooth logo in Bluetooth settings)
4. The app should find the device and start setup, but even though I was able to connect it in the end, it kept disconnecting/reconnecting constantly
5. While the app is trying to setup the device, go to your phone's Settings -> System -> Advanced -> Reset Options -> Reset Wi-Fi, mobile & Bluetooth (this will remove any saved wifi passwords and other Bluetooth devices, so make sure you know the passwords to reconnect)
6. Once I did this reset in step 5, the phone Bluetooth setting popped up and asked for the pin displayed on the Garmin device, but this still didn't stabilise the connection once setup, so I did the reset step 5 again and the actual Garmin Connect app asked me for the pin displayed on the device. Once I entered that everything was setup, synced correctly and the connection was stable all night and all day today.
I really hope this helps everyone as it was very frustrating for me after spending so much money on the device and not being able to use it. Good luck!!!