I don't think you can have multiple internet connections in full transparent mode. If Astaro is going to be managing multiple links, then I'm guessing you don't have a router, so why do you need transparent mode? FYI, there are other ways to use public addresses internally, if that's what you're looking for.
1. put the DSL modem on the Cisco router, if possible OR 2. change the internal IP of the Cisco router to a different network, and change the External IP of the Astaro to match. Then, continue to use your current IPs inside the Astaro firewall.