Is it another Apple watch you're switching to?

https://www.t-mobile.com/support/tutorials/device/apple/watch-ultra/topic/watch-settings/how-to-add-a-new-watch-to-an-existing-plan-when-setting-up-for-myself/10

No, im using the same apple watch

If it's an apple watch did you try the link. You need to unpair the original watch from the app.

I tried, but it isnt the same layout. It says “Your T-Mobile account is not eligible to enable cellular on your Apple Watch.”

You may need to contact Tmobile support using messenger on one of Tmobile’s social media platform like Facebook or Twitter.

Customer support is a joke, after 6+ hours on phone watch still doesn’t have cellular service. I cancelled my watch line because of it. AT&T is a bit more expensive but  worth it. I’m going back!