Smartphone/watch payment (e.g. Apple Pay) is functionally a fancy card. It doesn't need a signal to make a payment, ...
My experience differs with yours. Let me explain two different situations.
1. When I take my O2 Android mobile to church I turn it off before the service and on again after the service has finished. I once went to the supermarket after church and turned my mobile on in the supermarket. When I got to the checkout it would not accept Google Pay. I had to go home (it could have been church or somewhere else where I have wi-fi access) in order for Google Pay to do some sort of validation, after which I rushed back to the supermaket to pay for my goods.
2. I went to Ukraine, only 5km over the border, only 5km outside the EU zone my mobile has cover in, but when I came to pay I got an error message saying that I did not have a roaming package to cover data use. As it was, despite the error message, the payment was successful (the cashier confirmed that she had received the payment) and I later saw that it had used 56kB.
So my experience is that on turning on your mobile there is some validation that occurs through wi-fi and payments seem to require a little amout of mobile data.