Webapp iOS no video of other peer


We are developing a webapp where clients can call operators by using the jitsi lib API. The operator is working with Chrome while clients can access the website using Android or iOS.
While being in mobile network on Android the client can see not only him self, but also the operator, on iOS 12 the operator is not visible. Audio is working correct. If client is in a WLAN all works fine, even iOS displayes the operator video. Only if we switch to a mobile data connection using iOS the video disappears. The operator sees always the client in a good quality. So, the connection with webRTC seems to be established correct.
We tested it with different networking scenarios, also with different mobile carriers.

Do you have any clues for me?