Why can't I use my DynDNS hostname for firewall access?

There are 2 reasons why this won't work:

1. When the firewall rules are loaded the hostnames are converted to IP addresses (as otherwise every time the rule was evaluated a DNS lookup would have to be performed) and your IP address has changed since the firewall rules were loaded
2. TCP Wrappers (aka libwrap) checks the reverse DNS (rDNS) of the IP you're connecting to against the hostname. Since your ISP controls that it won't resolve to your DynDNS hostname but your ISP's hostname and so not match