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.