Why is my iPhone 6s keep saying no service..It was working fine for the past 8 years

You might want to ask that question of your phone company (carrier).

I'm looking at the MacTracker entry for that phone, and It appears that it had a lot of 3G support, as well as some 4G (LTE) support. Phone companies in the U.S. stopped offering 3G service some time ago. I don't know where you're living, but maybe the carriers are cutting off 3G service there, too, and your phone has fewer available cell radio frequencies with which to make a connection to cell phone towers.

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support