Hi
The functions that are available on Apple Watch when it is away from the paired iPhone depend on various factors including:
Features and requirements that apply under various circumstances are detailed here, including within the footnotes:
How to use your Apple Watch without your iPhone nearby - Apple Support
If you have an Apple Watch Series 3 (GPS + Cellular) model, then it may help to check that you have followed the instructions here for setting up and using the cellular feature on your Apple Watch:
If you still need help, then I suggest that, in the first instance, you contact your carrier for assistance with using the built-in cellular feature.
If the issue persists, then, depending on your carrier's advice, I suggest that you next contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: