It is an VDSL modem router, do you want to disable the Router function?
Can reset the modem, then add a bridge profile in the modem, then it is a pure VDSL modem.

I have activated the bridge modus but the router behind doesn't get an internet connection. With a different modem it works so it seems to be a problem of the vr900v. What additional configuration besides activation of bridge modus is necessary?

Is the DSL light? When you go to Advanced > Status, is the DSL status Connected?
And please make sure there is bridge profile in Advanced > Network > Internet page.
If yes, then there is nothing need to do. Can connect the PC to the modem directly and try dial up with username and password( for PPPOE) on PC. If it works on PC, then the modem is OK.

The dsl status is indeed connected and the bridge profile is activated. Nevertheless I don`t get an internet access neither by PC nor a router.
Which lan port must be used for connection with a pc or a router? I have tried all 4 ports without any success.
Once again: if I use a different modem or configure the vr900v as modem router everything is o.k..

After downgrading the actual firmware from vr900v(de)_v1_170515 to ...v1_161114 modem modus is suddenly possible, after upgrading firmware modem modus fails again. So it seems to be a bug in the latest firmware version.