Gateway + LAN problem

I run a cybercafe with a total of about 15pcs with a Vsat technology, my problem is that whenever i connect my server or some specific systems to the gateway, the gateway times out, when i remove the network cables it replies again, presently i can only browse with two pcs on the local area network, i have formatted all the systems in case of virus threat and scanned them as well but i keep having this problem, what can i do please, and there’s a good signal from my ISP.