Hi karenwoods,
Kindly ensure the following requirements are met:
(1) You are using a compatible iPhone model (iPhone 6 or newer) running iOS 11.2.5
(2) An eligible postpaid mobile plan (Combo, SIM Only or MobileShare) which you already fulfilled for SIM Only Plan
(3) Apple Watch Series 3 (GPS + Cellular) on WatchOS 4.2.2
You are encouraged to get their Apple Watch in Singapore. For watches purchased in other countries, please
click here to check if your Apple Watch model is supported.
(4) Latest carrier settings (31.1). On your iPhone, tap Settings > General > About and follow the prompt to update to the latest carrier settings.