How to connect cellular communication to my Apple watch?

Just to expand on what lobsterghost1 said, when you go into Settings/General/About you might see the Model number displayed as something like MKXY3S/A - that is a part number, not the Model Number. If that happens just tap on the number and it will change to show something like A2345 which is the Model Number to confirm cellular capability.

Identify your Apple Watch - Apple Support

There is also a very quick check available for the cellular versions. Look at the Digital Crown of your Apple Watch, if it has a red ring it is some version with cellular. Unfortunately it doesn't identify which region it supports so you still need to check exact model number (see below), but if the red ring is not present you know it does not support any cellular functions.

There is a full list of Apple Watch models and the different cellular networks around the world which offer cellular support. Please refer to Apple Watch - Carriers - Apple

Please also be aware that Apple Watches only have cellular function in the region for which they were designed. In particular, the North American models are not compatible with cellular networks outside North America. You can see this in the network support listing (above), where the Model Numbers are different of each region.

Are you sure your Apple Watch is a Cellular Apple Watch? Go to Settings > General > About and report back what the model number is.

7 is a Series. And each Series Apple Watch can be purchased as GPS only or GPS+ Cellular. You didn't do as I asked and post the actual model number, which will tell us whether your watch is GPS only or GPS+ Cellular.

Again:

Open the Settings App on your watch

Tap General

Tap About

See Model

Post the model number shown.

Thank you for adding to check for the red circle around the digital crown. I should have made it much simpler, by just suggesting that as an easy to know if the watch is Cellular capable.