Orbi as AP only able to complete AirMega setup on Guest Network

This is probably a unique situation, but I'm posting on the off chance someone smarter than myself can figure this one out.

I have my Orbi RBK50 configured as an AP behind an AT&T Pace 5268A Gateway (Gigapower Internet). The Pace is handling all routing functions and the network is set up as Class B (172.16.X.X).

Prior to setting up the Orbi, I would utilize two seperate SSIDs (5Ghz & 2.4Ghz) to manage those devices which required a 2.4Ghz connection. Conway's AirMega (Wi-Fi enabled HEPA Air Purifier) was one such device. I had no problem setting up the other couple of devices (LiFX Bulb & EPSON printer) which required a 2.4Ghz connection using the standard Orbi Wi-Fi channel. I hadn't turned on the Guest Network at all at this point.

During the setup process of the AirMega, an Ad-Hoc network is created to complete the setup. If I am on the standard Orbi Wi-Fi channel, when the Ad-hoc network is created it pulls an IP address in the 192.168.X.X range and setup never completes, which seems to be the issue since its not on the same subnet at that point. When I set it up as the Guest network everything works as expected and I can complete the setup.

Why would the standard Orbi Wi-Fi channel be pulling a 192.168.X.X address for the Ad-Hoc network in this scenario, and why wouldn't the guest network? I understand this is this routing table used if I were to have the Orbi handling the routing functions but it is set to AP, so shouldn't the Pace be assiging a 172.16.X.X to the ad-hoc network? I also find it strange this is the only device that has given me any trouble so far. I have it working on the guest network at the moment but would prefer not to use this work around and just have all devices on the standard network.

TIA