Calibre-Server and Port Forwarding Issues

Hopefully this is the right subsection for this topic? I realize this is not a 'router' oriented site, but perhaps some of you have gone through similar issues getting the Calibre-Server to work and have some tips!

Ok, have the latest manual bin install of Calibre on Mint Linux. Trying to setup port forwarding so I can share the my library. The Calibre-Server appears to work great, and I can test it using Port 8181. I can also log in using another PC on the local network, so that part appears ok.

And, using netstat, I can see that port 8181 is "Listening"

The problem does appear to be how to set up port forwarding so the server can be seen from the Internet.

I have setup port forwarding in the router to forward to 192.168.5.xxx:8181, which is the static IP address of the Linux PC.

Then using a web based testing site, I tested my external IP address and port 8181, but it says it is closed.

I then just just turned on full DMZ in the router for the Linux PC IP address. Still nothing using the web test site.

My setup is through a Megacable ISP modem > Asus RT-N16 Tomato 1.28 based router.

Any thoughts on where the problem could be?

My router WAN section shows:

Connection Type DHCP
IP Address 192.168.0.xx
Subnet Mask 255.255.255.0
Gateway 192.168.0.x
DNS 8.8.8.8:53, 8.8.4.4:53
MTU 1500

But those setting are different than my LAN settings which are:

Router IP 192.168.5.xxx
Gateway 192.168.5.xxx (points to router)

Is it possible the ISP modem is somehow blocking things because it is using a different IP for the router? I have so far discounted that because this setup, w/o port forwarding, has been going fine for years, although the Linux install is fairly new.

It must be something simple I am missing, LoL.

Thanks for any ideas I can try!