No connection via cellular

Since 2-3 days I have one big problem with my eufy 2C System.

When my phone/tablet is connected to WIFI I can stream the live-view, but when I am connected to the cellular network I receive most time the error that live-view could not be started. If I am trying it again and again, it sometimes works.
Some days before everything worked just fine…
It’s not a problem of my internet connection (Down: 100Mbit, Up: 50 Mbit)
I haven’t changed anything! Maybe there was an update?

I hope somebody can help me