Only thing changing DNS (Domain Name Server) server will do is change site lookup speed, not download speed.
You change the DNS servers in either the router if you have one, or the network adapter card in the computer if no router. There are no settings you can change in the Hughes modem.
Large speed swings are pretty normal on satellite and dependent on time of day. The real concern is when speed stays very low for a long time like days. Also download speed is dependent on the download server which may have a maximum setting.