SIM card iPhone

Your Apple Watch will work in cellular mode if it can't contact its paired iPhone (just as if the phone was out of range or powered down). If the watch can connect to the iPhone by Bluetooth it will try to route its communication through the iPhone, which will be a problem if the iPhone's SIM has been removed. Other non-communication functions in the watch will continue unchanged.

Use your Apple Watch without your iPhone nearby - Apple Support

I assume you will still be paying for the iPhone cellular plan, with Apple Watch add-on fee. Because if you stop that, your Apple Watch will stop cellular connections as well.