Answer Description
The most likely cause is that their iOS devices are simply too old to support Near Field Communication (NFC) which is required for Apple pay (and Android pay for Android devices). NFC support started in iPhone 6 devices and is generally available in all newer models. Apple pay does not require a cell or WiFi signal.
Wikipedia
Near-Field-Communication (NFC) is a set of communication protocols for communication between two electronic devices over a distance of 4 cm (11⁄2 in) or less NFC offers a low-speed connection with simple setup that can be used to bootstrap more-capable wireless connectionsNFC devices can act as electronic identity documents and keycards They are used in contactless payment systems and allow mobile payment replacing or supplementing systems such as credit cards and electronic ticket smart cards This is sometimes called NFC/CTLS or CTLS NFC, with contactless abbreviated CTLS NFC can be used for sharing small files such as contacts, and bootstrapping fast connections to share larger media such as photos, videos, and other files
Near_field_communication - Wikipedia, the free encyclopedia