This is a modem with a built in router I believe:
http://www.alliedtelesis.com/media/datasheets/rg613_ds_e.pdfIf this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems.
Double NATTo tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged.
If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ.
A preferred modem would be to have a stand alone modem with out a built in router.
The information on this ISP modem tells me that there is probably a problem with the modem and having an built in router already.