Same issue here, downgrade to 17.6 didn’t help!

However, I found a solution that worked for me: 1. Update your iPhone to the iOS 18 developer beta. 2. Then update to iOS 18.1. 3. Use a computer to downgrade your iPhone to iOS 17.6. This process successfully resolved the issues with the cellular network and call failures on my iPhone. I hope this helps!

iOS 18 beta 5 also works for me!