My new router arrived from Amazon today. I replaced my old linksys (which wasn't that old really) with this one, got the firmware downloaded, network set up, etc, and went upstairs to reset my panel, and it still had internet connectivity even after moving my router to before the XHS router. I did a reboot just to be safe.
When it came back up, had no issue whatever with it recognizing the internet. The weather ap works great, I was able to set and disarm remotely. Not sure why the XHS router wont work behind a Linksys AC router that's not even a year old but does on the Netgear NightHawk router that came out around the same time, but I'm happy for now.
Going to still have the 3939 installed though, it will allow me to pull off a computer from the rest of the main network, so that's a good thing atleast.