Could you try setting the DNS range on the DIR-655 to a range not being 192.168.n.n.
Think the outside IP and internal LAN are in same range, not sure but think this is an issue to get it to work.
Try something different like 88.n.n.n.
So your DIR-655 seems to get an IP from the modem (gigaset).
But it is itself via its DHCP releasing number in its default 192.168.0.n range.
Change this to be 88.n.n.n. as suggested.
After doing so you can reach the DIR-655 via its then default gateway 88.nn.nn.zz (probably zz will be 1).
Not sure if what i meant with Remote network but in the settings you will have to apply the 88.nn.nn.nn range to open ports or enable other functinality.