This happened to me. I was looking at my latest bill from T-Mobile, and I noticed that my watch didn't have any data use over the mobile network for the last two months. I hadn't noticed because the watch will transfer data via a phone connected via Bluetooth. When disconnected, the watch was connected to the T-Mobile network just without data.

After looking at some settings, I realized that I had disabled mobile data, not just data roaming, since I spent a couple of days in the Cook Islands back in May, which is among the few locations I've visited where T-Mobile's complimentary international roaming doesn't apply. While I had the watch in airplane mode, I didn't want to take any chances in case the device accidentally came out of airplane mode.

Coincidentally, I had purchased a Vodafone CK eSIM for my mobile phone, and I remember it wasn't working upon landing. Sure enough the APN setting from the installation wasn't correct. Luckily I was still connected to the in-flight Wi-Fi since the hotel where I was staying did not have complimentary Internet access.