RBR50 with AT&T Arris BGW210-700 - setup

Thank you so much for that explanation - now it makes perfect sense and I understand why I am having issues with the WiFi. To answer your questions:

1. The AT&T service is also providing telephone service.

2. The Arris is connected to a fiber ONT

3. The only thing I have physically connected to the Arris is the Orbi modem but there are other devices that are connecting to the AT&T network and not the Orbi network. I'm not sure if your question was do I have anything physically connected.  I have seen a screen showing devices that are connected and most are connecting to the Arris. I think this is what you explained with the double NAT - there are two networks. 

Would it be true that if a device is connecting to the Arris, the signal wouldn't be as strong as if it is connecting to the Orbi - (which is why I have the Orbi)? I have two satellites yet since this new setup (AT&T fiber/Arris), I am getting slow internet in some parts of our house. So, turning off the WiFi on the Arris should help this and is easy but turning the Arris into simply a modem is the complicated part. I have been having issues with finding my printer.

I have a question about "Connecting devices to both the Arris and the Orbi can also cause weird issues, like not being able to "find" a printer or a network storage device.  (That's another reason people try to get rid of "Double-NAT")." I don't understand this. I guess I don't understand how devices connect to a network. Is it true that those devices connected physically to the Orbi satellites like my garage door opener, Phillips Hue, and Solar, are connected to the Orbi network. Those devices like the Ring Doorbell and cameras that are looking for a network wirelessly, I can choose the network they connect to. Are they actually connecting to both networks somehow? Can I tell those devices that are connecting to the Arris network to forget that network and reconnect them to the Orbi which is giving me better range, turn off the WiFi on the Arris, and this may solve the problem? 

Is all of this still secure? 

I'm not sure I'm brave enough to try to figure out how to turn the Arris into a simple modem on my own and calling AT&T is a pretty awful experience. So, I guess a good question is: Can I have the Arris network just exist and tell all my devices to connect to the Orbi network, turn off the Arris WiFi, and this will solve the problem? 

Thank you for your patience and help!