apple watch 4 cellular doesn't get sms

Traditional SMS uses spare bandwidth on the voice band (GSM) radio of a cellular phone. Your Apple Watch has only a data band radio so it has to find the paired iPhone by Bluetooth or some other data link and route SMS through the phone’s voice radio.

You can’t reconfigure the watch without an iPhone, but if you have a temporary phone you might be able to login to your Apple ID using a computer and browser (or iPad) and find settings to remove the cellular number from Messaging. However this might impact iMessage direct to your watch so you will need to test and move with caution for unexpected consequences.

While you are in your Apple ID you should also review your trusted devices for 2 Factor Authentication, and if your deceased iPhone was on the list, make sure you still have a device capable of handling authentication codes.

I have some good news for you, the new phone setup might be easier than you are expecting. I had an equally traumatic phone replacement a few months ago when my iPhone 6S jumped into a 10m deep dock and it didn’t float. Fortunately I had made a new iCloud backup a few hours earlier. On my way to the Apple Store I collected a replacement SIM from my network’s street store, then around the corner to the Apple Store. After exercising my credit card (Apple Pay on the watch) the new SIM went into a new XS Max and I started the setup. After entering my Apple ID details it found my account (I forget if it asked for 2FA but if it did I think the XS got the code) and offered to restore my lunchtime backup during the phone setup. Than it found my watch config in the backup and offered to migrate the watch to the new phone. All totally painless, although a little time consuming with all the downloading. I think the only other thing I had to do was re-authenticate my cards for Apple Pay on the phone.

I think your watch will effectively do a reinstall, but it is pretty automated. All I can remember having to do is my watch passcode. My impression was that it probably downloaded WatchOS again and the WiFi was a bit slow so there was a bit of a delay, but I didn’t notice losing any data or functions on the watch or iPhone. Even my regular alarm times and settings came back. I think from memory I left the Apple Store with everything working and tested about 90 minutes after walking through the door.

Anyway, good luck, hopefully your experience will be as smooth as mine. It is really good to hear you also had a recent backup.

I believe so. It’s a complex area of technology and definitions, but I think you will find your watch falls over if the signal quality drops below UMTS (aka 3G). However the issue for SMS is not simply the available standards present on the air but the capability supported by the radio, where SMS is a system designed as part of GSM (basic level 2G) even if it is actually working a higher/later standard. I think I got that right, getting the split between system standards, codecs, protocols... it gets confusing so don't be surprised if I picked the wrong terminology.

So are phone calls on the Watch placed over the cell data band also?