Older versions of the GM OnStar hardware used 2G cellular radios and carriers have decommissioned those networks. Adapters are available for some vehicles to use newer networks.

https://www.onstar.com/support/faq/2g-network-sunset

Considering that OnStar spies on you even if you don't subscribe to the service it's much better to just leave it broken, or even to actively disconnect it in newer vesicles.