Apple Watch series 4 unable to pair with iPhone 6s

I have iPhone 6s (latest version 13.5.1) and the Apple Watch series 4, while I try to pair the phone app says to UPDATE or skip UPDATE & SET UP WATCH.. if I choose to update it is checking for updates for hours.. if I skip it pairs & unpairs in few sec and goes to the main menu..

I tried several times but it does the same as I mentioned above
Reset done for both devices , watch battery is fully charged, wifi network is good 
pls advise ..