i have put 50+ isp modems/routers into bridge mode.
i never had a case where all inbound ports are blocked.
sometimes, port 25,80 are blocked by default
a call to the isp and they will open those ports.
i know this is a basic question but the ip address on the wan interface of the ar300m, is it a public ip address or a private ip address like 192.168.0.xxx?