You don't need cell service for the health monitoring to work. The watch will monitor through Samsung Health installed on the watch - you don't even really need a phone, but it's easier to see and do everything in the health app, so you would need to have your watch and phone connected via Bluetooth for the stats to sync.
Just one more thing, though. If you are referring to the Samsung Health Monitoring app with the ECG function, you still don't need cell service, but you do need to have the watch connected to a *Samsung* phone via BT, as that feature only works with Samsung Phones and will not work in standalone mode.
