I've had a good experience with using EveryDNS http://www.everydns.net/
They are a free DNS manager and they let you mange up to 20 domains if i remember right.
Register with them, and point your namservers for your domain registrar to ns1.everydns.net, ns2.everydns.net, ns3.everydns.net, and ns4.everydns.net
Then configure your domains from the EveryDNS panel (eg A-records, NS, and MX for your mail).
Finally install their client so that your nameserver always point to your current IP.
No need to use ISPconfigs DNS manager in this case.