The "iCloud Private Relay" function from iOS 15 is intended, among other things, to ensure anonymous browsing on the internet. On the train, however, this function prevents your device from connecting to the ICE Portal. 

This function should be deactivated for the duration of use of the ICE Portal. You can check whether the function is switched off under "Settings" ⇒ "Apple ID" ⇒ "iCloud" ⇒ "Private Relay".