We have BM 3.6, SP2A and a remote Client behind a DSL Router (Sercomm

The connection will get established, but after 30 Seconds it says "VPN

Server is not responding, disconnecting", and then its over.
What is the reason?
Are there special requests for the router?
As far as I understand it, BM-VPN needs the ability to route the
SKIP-Protocol. But the people who sold me the router dont know what SKIP is and whether it is routed.