Not sure, but I think you need to look at how dhclient works...
It probably calls a script when it gets a new address - you need to hook
into this process.
Also, you could look at adding static interface routes... the webgui should
put them in the right place.
You are in a bit of an odd situation, which means you have to expect to do a
bit more of the work yourself.
Contact your ISP - ask them how to do it? Ask them and say you are using
FreeBSD - if they can help you there, then you can tell us what they had to
do, and someone here can convert to a mono config.