I hear port forwarding is hit or miss for some ever since they moved the port forwarding and other advanced options into the cloud. Does other remote management work like using the system to reset your gateway/modem? See here for instructions. If so, try a factory reset of the gateway and see if it works then. If not, you probably have a signal problem of some kind keeping Cox from communicating with your gateway.
Overall, if you use advanced options like port forwarding a lot, I suggest buying your own router. If you get Cox phone service, you should be eligible to swap the TG1682G to a TM3402 and use your own router. Or, if you get the gateway for free, you can bridge it. See here for instructions.