If you run leafnode as a server, then you should be able to add it into the trn client program just as you would add any other server.
There are no commands to resume an nntp connection. I haven't used a newsreader for a while, but as I recall it works similar to checking for email. You set the program to check for new "news" every so often. At that point, the connection is made, any new articles are downloaded, and then the client disconnects from the server until the next interval.
If your client fails to check when you have network connectivity, even though you lost it at some point, that sounds like a bug to me. Most clients, such as email clients or RSS readers, will try to check if the network goes down, and then will connect successfully when the network is back up. The client doesn't necessarily check right away when your network connectivity comes back up, it might be 30 minutes or 60 minutes, but the connection should be successful.