Could not activate cellular data network

I have been travelling to Japan and other countries. The phone (iPhone 6) worked fine for months before we left Switzerland. While in Japan, we used a local SIM card from docomo. For the rest of the travel, we used the Swisscom SIM card again. As we had other connection options, we left the roaming disabled. Phone calls, SMS reception and WLAN worked fine. Roaming was always disabled. Back in the Swisscom network, I can't connect to the cellular data network of Swisscom. Phone calls and SMS work fine.

I visited the Swisscom shop, they could not help me and sent me to Apple

I inserted another Swisscom SIM card that works fine - Same problem - hence the issue seems to be related to the phone and not the carrier

I reset the network settings without success as discussed here - no success

could not activate cellular data network

I checked for potential carrier settings updates - no success

iOS: Updating your carrier settings - Apple Support

Are there any other ideas how to get the cellular data network working again?