Site-to-Site with NAT - How do I set it up ?

Hi community,
I have two Astaros, between the two I want to setp up a Site-to-Site IPSec-VPN. The netowrk layout is:

Site A:
Astaro (Eth1, public IP) --> Internet

Site B:
Astaro (eth1, IP: 172.16.a.b/24) --> (internal Int., IP: 172.16.a.c/24)DSL-Router(external IP, public IP)
I am using the DSL-router there, because the Astaro is a virtual appliance which cannot interact directly with the ISP, and the DSL-router cannot be configured to operate in a "DSL-modem only" mode.

I set up the definitoins, gateways and policies by the book, but I cannot get the two sites connected. I think this must be because of the NAT-ing in site B.

Can anybody give me a hint on how to overcome this issue ?

Thank you very much in advance.

Regards
Tobias