Not familiar with the WRT300N, but on the predecessor WRT54GS router...at the Security>Firewall>Block WAN Requests area, I set the Filter Internet NAT Redirection to "Disabled" and it allowed me access from outside.
FYI, The other options on the Linksys Firewall area can be set as follows, as I know they worked for me:
Firewall "Enabled"
Filter Proxy "checked"
Block Anonymous Internet Requests "Enabled"
Filter Multicast "Enabled"
Filter IDENT(Port 113) "Enabled"
And I have the port forwarding should be set to an address outside of the DCHP range.
I am using a Clover DVR.