How do I add my cellular plan to my Apple Watch

See the 'Set up cellular on Apple Watch' section in:

Set up cellular on Apple Watch - Apple Support

Also, make sure that you have an eligible cellular service plan with a supported carrier. Your iPhone and Apple Watch must use the same carrier, and you must be within your carrier's network when you set up cellular on your watch.

See if anything here helps.

Set up cellular on Apple Watch - Apple Support

Use Apple Watch with a cellular network - Apple Support

https://www.verizon.com/support/knowledge-base-215386/

https://forums.att.com/conversations/apple/how-to-connect-apple-watch-with-cellular-att/5deffe06bad5f2f60604e88a