Cellular on Apple Watch
How can I set up my Apple Watch to cellular so when I go running I don’t need to have my phone on me
Apple Watch Series 4, watchOS 8
If your Apple Watch is GPS+ Cellular, you would need to make sure the cellular provider you use for iPhone can support Apple Watch. If they do, you can add a cellular plan to your watch. If your watch is GPS only, you cannot add a cellular plan. And you cannot use a different carrier on your watch, than you do on your iPhone.
See this link --> Set up cellular on Apple Watch - Apple Support