A better manual, and one that talks about setting up port forwards is located here: http://bectechnologies.net/wordpress/wp-content/uploads/2015/09/BEC-6800RUL_UM_1.3_2015_0906.pdf
Now on to the meat and potatoes… 
This router can be configured in Bridge Mode, so that your Asus router gets an external IP and not the radio. You want to connect your PC directly to the radio for any configs you do, and not through your Asus router. So that’s going to mean swapping cables around a bit, to get things set up for changing, but after that, everything can go back to how you have the cables run now. Just unplug the PC’s Ethernet cable from your Asus router and plug it into the PoE injector. The PC should get an IP from the radio’s DHCP server then and you are good to go.
You can find the setting on page 45 and is discussed in the section labeled IP Pass-through enabled. Before you just toggle this, though… Call your ISP again and ask them if they allow this to be done. They might have a MAC requirement for your radio, and if so, you may have to “change” the Asus’ WAN MAC to make this work. (This can be done in the Asus web app, too, so no worries if it’s needed… If so, let me know and I can find the relevant setting on the router for you to change, too.)
Let’s assume they give you the go ahead. Then all of your port forwardings done on the Asus router should work as soon as you restart it. Don’t just flip this on without checking with them, because you may not be able to get back into the device without going out and resetting it! (Manual reset with the button on the device.) Doing a reset might lose all of it’s configs, and then you may need to have a tech come back out to configure it!
If they tell you “no” then you are going to have to do some different config things on this device. Pages 70 and 71 talk about port forwarding, so read up on this section fairly well before you go into the radio and start changing things.
The only IP that you would need to forward to here is to your Asus router, which should already be assigned to it. (192.168.1.x). I couldn’t find a place in this device’s configurations for setting up a static, but that’s not an issue if the router is the only device you have on it’s network.
Make sure of port numbers on your port forwards! If you don’t want to change from 32400, then don’t. If you tell Plex to manually map a specified port and it’s not 32400, then only the Radio’s external port is going to need to match it. Example:
Radio port mapping:
Ext port 21212 to Asus router IP on port 32400
Asus router port forwarding:
(from radio) 32400 to PMS IP on port 32400.
This should then work as well as configuring in Bridge Mode… Let me know how this works.
(Basically by not being in Bridge Mode, it means you are going to need to do all port forwards 2 times. Once on the radio and once on your Asus router. So if you set up your own web site on a PC in your network, you are going to need to set up a port forward on both devices to support that web site. This should be the only issue you might have if you can’t run the radio in Bridge Mode.)