Multiple static IP's on FVS-318N
It works, and I have seen it work countless times. However, I have also seen the feature have issues on devices, can't recall if ever on the FVS318N. Generally it falls in to a few categories:
1. ISP isn't actually sending traffic to that IP and have not configured a route on their end, if a capture shows nothing and you're sending traffic to that IP to test, then you'll know this is the case immediately.
2. The ISP wants a Proxy ARP which our devices historically did not support, although may currently(I'd have to check); rebooting the modem, removing the battery if it has one and waiting a moment before putting it back in, generally fixes that outright.
3. It is working, but the traffic doesn't appear to be; running a capture will show what is happening and is always recommended for verification. Software issues look like this generally, where the network and all is fine but something isn't configured right elsewhere.
4. ISP isn't actually sending traffic to that IP and told you the incorrect range of IPs, and you have to call them to verify what it is. (rarely, but occurs enough that I've seen it a few times)
5. Old firmware bugs. Stable currently doesn't mean it always was, and I didn't see which one it is, mentioned above. The latest I always recommend if having issue. If you're fine and stable, never change. If it aint broke, don't fix it is always sound advice.
6. Something else/fluke/bug. For that we would collect as much information, config file, remote access if possible, and work with our Engineering team to debug the issue and figure out what is going on. (rarest, as port forwarding part of the unit is the most stable and least changing feature)
Either way, I'd check the first 5 before heading to number 6. Also, captures will give you a lot of information. Forward ping instead of your service for testing, or send that IP to an HTTP server you have and test with your phone. All this should at least give you an idea of where the issue truly is, and if it is us, then having all of that information on hand is the fastest way to go forward. More info helps everyone involved. Right now we're just throwing out ideas and things we've commonly seen. 
As far as how you have it configured, other than the service, it looks fine, although now the devices have the secondary addresses on newer firmware. I'd have to check when they added it but I thought it was the 4.x firmware codebase, which would imply you're on the old 3.x firmware, but again that's my memory and I could be wrong.
Edit: Also, in seeing a later reply, you do need the outbound rule bound to that IP as well.
