Hello harshp94,
Thanks for asking the Apple Support Communities questions about using your Apple Watch while it's not paired with your iPhone.
This article talks about using your Apple Watch without your phone nearby, but also states some functions require the phone to be paired:
How to use your Apple Watch without your iPhone nearby - Apple Support
If your iPhone and Apple Watch Series 4 (GPS + Cellular) or Apple Watch Series 3 (GPS + Cellular) are both connected to a cellular network, your watch can do everything that it normally does, even if your iPhone isn't with you.1 When your watch connects to a cellular network, 
appears in Control Center.
If your watch is connected to a cellular network, but your iPhone isn't nearby, your watch can do the same things that it can when using Wi-Fi.
Learn how to set up cellular.
To receive SMS from third-party apps, MMS from third-party apps, or push notifications from third-party apps on your cellular model of Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.
As for the cellular plan payment with your Apple Watch, that is up to your provider. Thanks and have a wonderful day.
