You will need to contact AT&T technical support and have them submit a ticket to reset the e-sim on the watch. Do not waste your time going to a store or talking to a customer service rep. They do not have the ability to reset the e-sim. You may even have some of them saying crazy things like needing to open the watch to reset the e-sim. If you hear someone tell you this, they do not understand how the e-sim works. I had to do this with a Gear S2 Classic I bought on Swappa a few months ago, and then did it again when I moved a phone number to a different watch. You should be able to see the previous phone number on the watch, which will make it even easier to reset for you if you tell the tech support person what the number was and what number you want added to the phone if needed. If the seller already removed service from the watch, it will make it easier as AT&T will not think you have a stolen watch. The process may take a few hours or less to do, depending on how many service tickets are pending in the tech support department.
Just FYI, the e-sim does not reset just by removing service from the watch. It is permanent unless you get AT&T to reset the e-sim or switch carriers and have T Mobile activate service on the watch. This whole process of having to reset the e-sim is a major flaw in the whole e-sim setup.