How can I add cellular to my Apple Watch?

No because the cellular watch has a built in E-sim that uses the phones number so that you can use it without your phone being with you.

GPS only watch doesn't have an E-sim which is why you have to have your phone with you to make calls and send texts.

If you haven’t bought the apple series 5 GPS + Cellular, and only the Apple Watch series 5 GPS, you aren’t able to use cellular on your Apple Watch. Have you got the cellular version though?

No, it must be purchased. You must have your iPhone in a close proximity to the Apple Watch in order to reply to messages and make phone calls in that case. Good luck with your Apple Watch.