The way "passive" payments work is that your phone generates a new QR code every few seconds, same way that those one-time password authenticator apps work. The vendor scans your code and charges an amount, then you get an SMS receipt (or confirmation request, for larger amounts).

The other way around - the internet-lacking vendor has a static QR printout (or one-time code) which the buyer scans, keys in the amount, then pays with their internet connection. Vendor gets SMS receipt.

Either way, I do think one of the two parties needs to have internet for a payment to successfully go through.