TL;DR UPnP issue maybe? It is a mystery.

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I've got a Routerboard 750GL siting behind a ISP supplied FIOS router. Frequently systems connected to the network will report that they cannot connect to the internet even though they can connect to the local network or the wireless access point. Normally if I can force the device to connect to a specific address then suddenly the device can tell its on the internet. I've also got several broadcasting devices that intermittently think they are not connected to the internet too. This leads me to think that my devices are trying to use UPnP.

This is for a home network so the router also provides via the coax cable DHCP and connectivity for cable boxes. After enabling UPnP on the router and adding the internal and external interfaces the issue did not go away. I'm using a Chromecast and a Wireless scanner as testing clients. I also enabled a DMZ zone from the FIOS router to the RB750's external interface. Once I did that I got the connectivity to be restored for the 2 test devices. Although it didn't last long, both were reporting offline within 20 minutes.

Is there something I can test? What about bridge mode for the FIOS router to external RB750 interface?

Any help would be great. I'm really trying to learn much more about networking and these "issues" are great learning experiences.

Thanks,
Jason