NIS slave server across a router?

I am trying to use NIS on two networks connected with a router. NIS is
running fine on one network with a master server and a slave server. I
would like to have a slave server running on the second network.
 
I have been able to get a NIS client running on the second network as
follows:
	In /etc/sysconfig:
		nis_clientflags="-s -ypsetme"
		nis_ypsetflags="<hostname of NIS master>"
 
But this is not sufficient to run a server across networks. The server
will run, but it does not receive any NIS maps from the master server,
/var/yp/iaeunguessable stays empty.
 
I could run a NIS slave server on the router (it is a FreeBSD machine), but
I'd rather not do that. Is there another solution?
 
The NIS master is running FreeBSD 2.0.5, the slave server to be is running
FreeBSD 2.1.5. The servers also use nis_serverflags="-p667".
 
Thanks for any help.
 
Regards,
Edwin de Graaf