could be that the router won't serve bonjour.. Here is a list of services and corresponding ports - do a search on that page for 'bonjour'
could also be a problem with the 'server.pem' file
there are also replacement files available on the net (google 'server.pem filemaker')
Also note that bonjour will not cross subnets. So if those other machines are on a different subnet than the host machine the Open Remote dialog box will not show any files.
There is also a setting under sharing, to not displace files under open remote dialog. Make sure this option is not checked.
I have not used FM12 for some time, but was there not an issue with the security update towards the last few updates which I think required SSL to be switched off or the only show files available to this user needed to be turned off.
I am sorry I can't be more specific, I vaguely recall having issues with FM12 clients accessing FM13 hosted files and it was something to do with the SSL security update which required the latest fm12 update and some setting changes.
The problem was due to updates in our antivirus that had reset all routers to public and the newest router settings required that the 5003 port be opened with in the firewall of each individual computer trying to connect to the host file.
Going to the firewall setting of the individual computers and resetting the router from "Public" to "Work". This was all that was required for some routers.
Other routers required port 5003 to be designated as open on "Work" routers with in the firewall of the host and connecting computers.