RBR50 with AT&T Arris BGW210-700 - setup

I also have had a similar setup for a while, but was trying something and need some help. I haven't yet changed either device according to the Options 2 and 3 given to the OP.

I have the same models, fiber, with voice but it's through Sonic (using ATT's fiber). The voice has a separate box connected to 1 of the 4 Arris ethernet ports and not the Voice port.

I don't remember whether I had changed anything when I first set this up, but had to reset the Arris when my internet was out. So both wifi's were on, and both were DHCP servers at least after that. The Arris gave the Orbi a 192.x.x.x IP address for its gateway, as well as for other network devices. The Orbi uses 10.x.x.x also for other network devices. 

There are two reasons I got the Orbi+1 satellite, and neither of them have been solved. I have a small older one-story house (old electrical, but that's another issue). Besides laptops, phone/tablet, wireless printer, and another old networked printer that I had set up to be able to use over wifi (but is out of toner so may give that up), there are the below:

1--I have two wifi Roku devices, one Express (which only can do 2.4GHz), and a Streaming Stick (can do either, but there's no way to designate which to use on the Roku). 

Supposedly having the fiber should have solved buffering issues, but at least one channel sometimes says its buffering because it's too slow, even if my cloud backup isn't actively backing up.

2--I thought I'd be able to connect USB hard drives to the USB ports on the Orbi, to replace a TP-Link W8980 which can enable me to use those drives wirelessly (and the interface allows me to see them and disconnect them in the software). It's an old ADSL modem/router, and I was only using the router part, but it's older and I can't use my newer 4Tb drive on it (it can only go to 2Tb). That doesn't work, and also doesn't work on the Arris. I thought I'd try connecting the TP-Link via ethernet to the Orbi just to see if that would work with the smaller hard drive, but I couldn't see it anywhere (in Orbi or as the following). With the old network setup, I could access the shares by entering \\192.x.x.x, the IP address of the TP-Link in Windows Explorer, and both volumes would show up.

So, I have three things.

1) Not sure which router should be the DHCP server, and which set of numbers to use where (192.x.x.x or 10.x.x.x or both). I was getting confused by the Arris giving the Orbi the 192.x.x.x for gateway then when I tried to use the same 192.x.x.x on the Orbi for DHCP and not use the Arris, it said I couldn't because of the gateway.

At the moment I can't get to the Orbi unless I connect directly with a cable (I tried some ways to have its IP different from 10.x.x.x but it didn't seem to work).

2) Is it possible to force the Roku streaming stick to use the 5GHz band? 

3) If I had the rest set up properly, would my idea of connecting the TP-Link (or some newer similar device that can handle 4Tb, why I was testing) work via ethernet to the Orbi?

Thanks.