why is my cellular not working on my watch?

Your description suggests the Apple Watch is correctly configured and probably working ok. Be aware for SMS using the watch away from your phone, the phone must still be powered on and connected to the cellular network. It doesn’t matter if it is miles away on your coffee table, but it has to be powered up and accessible. Don’t ask me why.... I suspect something obscure in eSIM technical standards.

Branta, I'm sure you mean well and thank you for trying to help. Unfortunately, your comment is incorrect. Your iPhone does NOT have to remain powered on when fully detached from it. If you have an LTE enabled AW, it will work independently.

Hi Branta, it's not a theory. I understand there's documented instructions (thank you for displaying that) that claim your iPhone must remain powered on for you to get notifications and be able to make calls when disconnected from the iPhone, but that document is odd and incorrect. Based on the document, I can see why you made that first initial comment. The Apple Watch has a baked-in SIM card....meaning, when it's not connected directly to the iPhone via bluetooth, and WiFi on the Apple Watch is disabled, the watch uses that SIM card to connect/authenticate to your cellular plan, independently. I have tested this by powering down my phone and using the Apple Watch without fail to make calls, send/receive text messages and stream Apple Music. The built-in SIM card manages the authentication needed to connect and use the cellular LTE plan independently. I believe this may not be affecting all Apple Watches across all cellular providers, but I assure you, I began having my issue immediately after I installed Watch OS 4.3. Perhaps in my case, the new OS has created a hand-off bug between the Watch and T-Mobile's cellular network.

I refer to https://support.apple.com/en-gb/HT205547 and specifically footnote #1,

<begin quote>

To receive SMS, MMS, or push notifications from third-party apps on your Apple Watch Series 3 (GPS + Cellular), your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.

<end quote>

I just tested your theory here and I’m not convinced. I left my phone on iOS 11.3.1 ( in the usual state with WiFi and by enable) to charge in my hotel room, and walked out to the front door. In 3 tests the watch (OS 4.3) connected every time. Tests 1& 3 I saw the watch drop to disconnected then within about 30 seconds I had 4 green dots. I. Test 2 took a little longer, about 3 minutes Was needed before I got a watch cellular connection.

This is was not a scientific test with only a few attempts at one single location so,it isn’t perfect but it does suggest there is not a systemic problem in all Apple Watches. if the problem was software as you suspected we would probably have complaints into hundred if not thousands.

I have good news and bad news. The good: you're not alone. The bad: its the LTE antenna NOT connecting to your cellular plan. I've noticed my Series 3 Watch with cellular started doing this right after I installed the latest Watch OS 4.3 (15T212). To confirm we have the same issue, turn off Bluetooth on your phone, then on the Apple Watch itself, turn off the WiFi...this should/suppose to force the Watch into complete independent/cellular LTE mode. Meaning, you should be able to text, call, stream Apple Music and update the Weather, while being disconnected from your phone and any nearby remembered WiFi networks. What you will notice is your Apple Watch will continue to search for a cellular connection and never actually connect to your provider (best viewed on the Explorer Watch Face with the four dot cellular indicators). Apple is pretty hush about this, but I've seen several people on other post reporting the same issue. This issue ****** me off because I'm paying a $10 monthly plan fee to T-Mobile and the **** functionality on the device is busted! Apple replaced my Watch once already, and the issue was still present on the new swapped Apple Watch. I spent a week without my device, and it's a neglected and unacknowledged software issue.

Aahh....I just realized what this footnote (link) says. The key word in the footnote is "third-party" apps. In other words, the native SMS, MMS, native weather app and calling features all work on cellular regardless of whether your iPhone is powered on or off. If you have an app for example, WhatsApp, Google Hangouts, or FaceBook Messenger; this document suggest that those particular third-party (non-native) apps will not function unless your phone is powered on (does not need to be nearby). This I can confirm is true. I tested Facebook Messenger, plus WhatsApp and could not send/receive messages while connected to LTE cellular connection so long as the iPhone was powered off elsewhere. To that point, you were correct. However, all native Apple apps work without the need of the iPhone staying powered up and connected to a cellular or WiFi connection.

I agree figure you didn’t have LTE enabled and were in a low signal area