Hi Phil,
I'm Sumit, here to answer your query at the Microsoft Community.
Apologies for any inconvenience you are experiencing. I am happy to help you today.
The page you are trying to add your Android phone to isn't working lately.
To link your Phone to your PC, you need to use the Phone Link app - the Android devices page would show your device when Microsoft fixes the page you are trying later on.
https://www.microsoft.com/en-in/windows/sync-ac...
Android:
https://play.google.com/store/apps/details?id=c...
Sign in with the same MSA you are using for the Phone Link app in Windows.
Another bonus feature if you want to show your Mobile in the File explorer, just go to Settings > Devices > Mobile Devices.
I hope this information is helpful. If you have any further questions, please feel free to reach out to us. Good luck!