I had a Series 5 which was previously paired with an iOS 13 iPhone. When I attempted to pair with my iOS 14 iPhone, the Watch app was requiring me to update the Watch OS before pairing. I was getting the "Unable to Connect to the Internet" message regardless of all the router troubleshooting previously mentioned. The trick for me was attempting to pair it with my wife's iOS 13 iPhone. I stopped when it told me her iPhone iOS needed updated before I could pair with the Watch. When I attempted to pair again with my iOS 14 iPhone, it worked! There seems to be an issue with being previously paired to an iOS 13 iPhone and needing the Watch OS to be updated before pairing with an iOS 14 iPhone. I'm surprised this hasn't been patched yet.