This is my setup.

Modem - Linksys Router running ddwrt - grandstream.

I login to the grandstream set the static IP to work on my network. Went into my router and set port range forwarding for 5004 - 65000 UDP to the static ip.

Plug in it and everything works!

The reason I put it behind my router is so I don't have anything between my computer and the internet. I don't like having devices between my connections because of legacy. Don't have to worry about the grandstream lagging and not being able to transmit all of the traffic. Plus I set (QOS) Quality of Service to allow the port (on the router) to have priority over everything else.