Activating cellular on an Apple Watch Ultra 2 that I cannot pair to an iPhone.

I hope somebody might be able to help with this. I am being driven mad by it. I purchased a Ultra2 aiming to pair it with my phone and use the cellular. My phone cellular provider is Three on a business account. This means I cannot pair to that account. How on earth do I now get cellular onto the watch? Can I purchase an esim and add it to the phone and then pair the watch to that?

[Re-Titled by Moderator]