You will need to have a routable external ip...cable, fiber, dsl, will be no problem, satellite, cellular, can be a problem ...you also need to either setup vpn (most secure) or port forward.
vpn is no out of the question...
dmz is not related to vpn..if you turn on dmz, then plug another router into yours and disable wifi on the att router it will work just fine...you need to start doing research...or roll the dice and port forward.
Almost every router and firewall allows dmz, even business units...you are missing the point. You place setup a SECOND router that has a proper firewall and vpn, BEHIND the att router....google your routers model number and bridge mode and see what comes up
There are several threads discussing asus routers...
I have various units depending on location and need. Most are business that allow for vlans. I generally use zyxel and now am toying with untangle. For your level that would be too much. Go with the asus...
I dont do anything that is technology related. I have installed hundreds of cameras and managed many networks in my lifetime. I can ascertain your level based on your posts that is why I am giving you this specific advice. You would be lost trying to use a business gateway..possibly making things worse by allowing unwanted traffic.
Second that. Very easy to setup VPN.
Forgot to add that I did try that for the BI pc using windows network setup and while the PC became static, I had some intermittent network/ internet access. May be it has something to do with the way I left DNS or gateway dynamic etc. In any case In my older tomato USB on asus rt-n 16 router, I setup up static on the router and it was very stable.