my setup is cablemodem -->router--->grandstream and it works fine.
I have stuff running on the router (ddwrt&optware) and all works fine.
I had to change the grandstream to a set ip as not running a dhcp server (do not need someone parked on the street hacking the internet). on the grandstrteam lan port is empty as have no need for anything hanging off it.
I had to call support to get it working as they had me change a setting in the advanced tab of the grandstream.
all has worked great and I still have my media server running off the router with no issues.