You mean it does not need any wifi or network to use? it's a naked phone with a plastic case. was working well last month and prior, only recently start to give problem.
samsung pay does not require internet connectivity to function, but the number of transactions that can be completed when samsung pay has no internet connectivity is limited.
Meaning, samsung pay app generates a few of the tokens and standby in the app for use whenever it has connectivity to the internet, so that in the event you don't have internet, these tokens can be used readily..
if you think hardware wise got issue, there is actually the service centre which can help you confirm.. Both MST and NFC aspects can be tested as they have a physical tool for it..