Try this as a workaround, it worked for me.
Add the IP address of your routers external interface (in this case 18.104.22.168) as a secondary IP address on your default NIC.Now the important bit.
Give the secondary IP as tight a subnet mask ask you can. I used 255.255.255.252 on mine but 255.255.255.255 may also work.
The reason for the tight subnet mask is to prevent your PC routing packets destined for your ISPs wider IP range out through your secondary IP. Any traffic leaving your PC that way wont go anywhere as your primary default route won't be available via that subnet.
Therefore having a broader subnet mask could prevent you from connecting to the rest of the ISPs network.
I also have Multihome set to my PC's primary IP address.
I edited "america's army 3 dedicated server\AA3Game\Config\DefaultGame.ini" and set sMBSExternalGameServerIP= to my routers external address also.
I'm not sure if this overrides Multihome, but it does mean that i get the external addess referenced by MBS correctly.
Log: INFORMATION: MBS configuring GLOBAL server mode with external IP "x.x.x.x"
What all this does is it tricks the server process into thinking that the IP you will be presenting to the world is local to your machine, therefore allowing it to continue.
Binding all the AA server services to you primary IP using multihome means that you wont actually use the secondary IP for anything anyway.
So, give it a try.
Like i said it works for me.