Make sure you have copied the certificate of primary server to all the secondory server.
This will establish the communication between servers. Then only you can issue any command onto secondary servers.
If a 64Kbps line response from secondary servers will be always slow....
Secondary servers have to download 50MB updates. In 64Kkbps link updation will be slow.
Its better to move all the clients to one server. Since all the clients will be under one server and gets incremental updates it wont load the network.
(You can use this link to configure update clients by net or subnet. This will reduce the network traffic for one branch