When you log into the Quantum Gateway Router, what is the WAN IP of the device? You can find this under the status page. If the WAN IP is an Internal IP say 192.168.x.xxx then your modem is not in bridge mode. This means you are engaging in “Double NAT”, which is a router behind a router. This would explain why Tablo Connect does not work even after forwarding the correct ports.
With two devices, it is very likely you will need to put the modem in bridge mode yourself. This can be done by plugging the Ethernet cable from the modem that goes into the WAN port on the Quantum Gateway Router into a computer (aka laptop with an Ethernet port). For more information about what Double NAT is, see the links below:
http://www.practicallynetworked.com/networking/fixing_double_nat.htm
http://inmethod.com/forum/posts/list/908.page;jsessionid=71DD206BA081E75FC2157F10FC63781A