"Your Phone" won't connect to phone after motherboard replacement

I've had the Your Phone app running for the last few months on my Windows 10 Pro PC, connecting to my LG V20 phone (Android v8). Last week my motherboard failed, so I replaced it and restored my Windows setup to a new SSD while I was at it.
Now the Your Phone app won't connect to my phone - it says: "We can't connect to your phone. Make sure that your PC and phone are connected to the internet".
The name of my phone (V20) is still displayed in the top left corner of the app Window. Both PC and phone are connected to the internet (via the same router). I have tried:
- Uninstalling and re-installing the Android app (and re-adding the necessary app permissions)
- Resetting the Your Phone PC app
The Android app says "Check your PC", and tells me to select "Send notification", but that option isn't available anywhere in the PC app that I can see.
Can anyone advise on how I can get re-connected to my phone?

- - - Updated - - -

...one step forward...one step back...after another re-installation of the Android app I was able to get as far as sending a notification from the PC app, and selected "Allow" when the notification came up on the phone. Now the phone says "Your phone and PC are linked", but the PC app still says "We can't connect to your phone"
????

Ztruker said:

UN link your phone from your PC then start over.

Click on the Search icon (magnifying icon) and enter Your phone. Click on Unlink this PC.

Try again. Maybe even reboot.

I don't see a search icon anywhere - are you talking about the phone app or the PC app?

Oh OK I managed to find the Your Phone app but I don't see an "unlink your PC" option anywhere

So I tried the "unlink phone" option to unlink the PC, and it seemed to remove the phone. I then selected "Add phone", and it immediately reinstated my V20 phone, and said "We can't connect to your phone" again (it seemed to have remembered settings from last time, as I didn't have to do anything else to re-connect the phone).
???