SSH Conection does not work with VPN?

I installed Hass.Io and the SSH server add-on.
After that I can connect with eg. Putty to my pi.
But if I use a VPN connection (VPN from my FritzBox router) I can not connect.
I get this error: “could not resolve host name”
Is it not possible with und VPN? I tested the VPN connection with the all in one raspberry pi installer- and there I can connect over VPN to the pi.

Thanks for your help!