Warning, T-mobile smartphone plan with a separate number does NOT work on Apple Watch

Purchased a separate line for my Apple Watch Ultra from T-mobile while my iPhone is on Mint mobile. I thought that this would give me a $5 / month data over LTE for my watch without being forced to switch carrier for my iPhone (Mint mobile does not support Apple Watch).

T-mobile did have a product for it, they did sell the line to me, they activated the line, they gave my watch a phone number, and they confirmed that my watch is communicating with their network.

But it does not work. Watch never managed to communicate over T-mobile network.

I spent total of 6 hours on T-mobile support, half of the time their reps researching why my order was stuck in delivery queue and half of the time with their tech experts trying to research why the watch does not work. Finally one of the second-level tech experts connected Apple Support tech expert to the same call. After researching Apple internal documentation she confirmed that Apple Watch can only work on same courier as iPhone. No-one at T-mobile knew that it would not work. Apple expert did not know that before spending time researching their documentation.

Just posting this here in hope that it might save someone 6 hours in tech support ****. Or a purchase of LTE model when on carrier that does not support Apple Watch.