"I have an unused China Mobile SIM."

No, you have an unused China Telecom SIM. You know, China Telecom, the one which runs a CDMA network that doesn't work with most foreign phones.

Besides, this website -- which may or may not be right -- says of China Telecom SIMs, "Only purchase these cards in official stores! They can’t be activated anywhere else, so a card purchased from a street vendor will be useless."

https://www.vpnmentor.com/blog/best-...-can-get-them/