I think the brands does not affect MR3420 work as a bridge or repeater.

Yes, thanks for your add.

Yeah, it is safer to use VPN service.

You should ask TP-Link's support department to help you solve your problem. Do you have the S/N number of the D20?

You can check the "ftp (Via internet)" option under Advance>USB settings>sharing Access. Then you can connect to your USB device remotely by entering ftp://x.x.x.x:21.

I am afraid this firmware cannot be installed on the VR900 German version. VR900 Annex B version is VR900v and the link you provide is for VR900. VR900v is a different type of model from VR900.