I did it!!! I did it!!! like everyone I was also stuck with a hard reset watch Series 3 and iPhone 11 (my IT ego really hurt!!) scourging the internet and trying everything, I came to a realisation that the issue for me was not of region or backup failure or anything rather a direct connection to internet!! I normally use a wife mesh at home and after getting very desperate and some comments from internet talking about strong Wifi connection I removed the Wifi Mesh and connected by phone directly to the base router Wifi and used the below manual DNS 1.1.1.1 and 1.0.0.1 which seems that Apple server was able to respond in time that my iPhone App accepted the update!!!! So the issue seems to be the Wifi or network lag in response from the Apple server!!! I faced a similar problem when I was setting up my macbook pro for the first time and the macOS download kept failing but after many attempts it finally completed the download from the internet. Hope this works for others!!! Reduce internet hops in your wifi and use manual DNS for faster response times. Oh Yes make sure your watch is on a Apple Watch charger not a third party charger.

None of the above worked for me! Did anyone find any real solution!!! I have Apple Watch Series 3 and iPhone 11 Pro Max. Every sync stuck at "Unable to Check for Update"!!!!