Is QOS enabled?
Did you set it up yourself or did it come pre-configured?
You should probably factory reset it, and then the only thing you need to do to get it up and running is creating a DHCP pool.
First port is WAN, second port is one network, third port is another separate network. Should plug your Modem into port one, and then a switch to port two and leave port 3 off/disabled in the settings.
Good luck friend