Hekaveb Have you tried turning if off and manually selecting? You'll notice that all the other networks are forbidden. Your SIM card is to be used with your network provider, other providers will block it.
The exception to that is when you are roaming in another country, you often can switch to a number of mobile networks. If you were having problem with a particular network for roaming, you could switch to another. That's the only time I have ever used this feature in Android (it's a standard Android feature, nothing to do with GrapheneOS)