I've been using DynDNS for years as my DynamicDNS provider. Recently, they've taken another step towards eliminating free users (you have to log in every 30 days, not just update your IP). I also believe they no longer accept new free accounts.
This prompted me to look into the other providers that m0n0wall supports, and this is what I've found. Currently, none of the providers that m0n0wall supports are free. Some are pretty cheap, but they've all added some sort of fee to their services. There are other providers out there that are free, but m0n0wall doesn't (directly) support them.
There have been a few posts throughout the years asking if 'service xyz' can be supported by m0n0wall. Most of these threads have no response. From what I can tell, m0n0wall uses a package called 'ez-ipupdate' to provide the Dynamic DNS service. This package doesn't appear to have been updated since 2002. I took a look at pfSense, and it appears to support most of the services that have been requested over the years. As best I can tell, pfSense appears to be using a PHP script to handle the Dynamic DNS service. What would it take to port this over to m0n0wall? I know m0n0wall is open and I'm always welcome to dig in and change this myself, but alas, I am not a developer and don't have the skill for this.
Summary of m0n0wall-supported providers:
Service Status
DynDNS Not Free
DHS Not Free
ODS No New Users
DyNS Not Free
HN.org No longer exists?
ZoneEdit Not Free
GNUDip Requires your own server
easyDNS Not Free
EZ-IP No longer exists?
TZO Not Free