This is certainly an interesting issue you're facing, There's a few possibilities as to the source of the issue but we'll just need a bit more information to assist in getting to the bottom of it. I'm curious, can you confirm for me if your apple watch has remained active since 2019 or was the line cancelled at any point? Was there a change in phone as well during that period?
I changed my phone service from Fido to Rogers in Jan or probably Feb in 2019, and remained since then.
I bought my Apple Watch Series 4 in Aug 2019, and registered my smartwatch service in Sep 2019.
Since then, I never changed or terminated my phone or my watch services.
Before COVID-19 outbreak, I usually jogged with my watch alone. I can use it to receive messages and calls, to listen music, and to pay with Apple Pay. However, I don't use LTE function from last year until last weekend. I don't remember exactly how long I don't use, but I am sure at least half a year. From that time, I always keep my watch updated to the latest version, but I don't test if LTE function works well after updates. (I don't hear any news about LTE bugs in recent updates from Apple.)
Thank you very much. I am glad to hear any advice.
Thanks for your reply! If you don't get notifications, messages, or calls on your Apple Watch, your Apple Watch and iPhone might not be connected. When your devices disconnect, the red iPhone icon or the red X icon appears on your watch face. When your Apple Watch connects again, the green iPhone icon appears.
First, bring your Apple Watch and paired iPhone close together. If you see that the devices are disconnected, try the steps below.
Try to connect your Apple Watch and iPhone again:
Keep your Apple Watch and paired iPhone close together to make sure that they're in range.
On your iPhone, make sure that Airplane Mode is off and that Wi-Fi and Bluetooth are on. To check, open Control Center.
Restart your Apple Watch and iPhone.
If your Apple Watch still can't connect, unpair it from your iPhone, then pair your Apple Watch and iPhone again.
How to unpair your Apple Watch from your iPhone:
Keep your Apple Watch and iPhone close together as you unpair them.
Open the Watch app on your iPhone.
Go to the My Watch tab and tap All Watches.
Tap the info button next to the watch that you want to unpair.
Tap Unpair Apple Watch.
Choose to keep your cellular plan.
If you want to pair your Apple Watch and iPhone again, keep your plan.
Tap again to confirm. You might need to enter your Apple ID password to disable Activation Lock. Before erasing all content and settings on your Apple Watch, your iPhone creates a new backup of your Apple Watch. You can use the backup to restore a new Apple Watch.
After your Apple Watch unpairs, you'll see the Start Pairing message.