What modem/router can I use as Bridge to connect to the AT&T gateway? Any recommendations?

Alright. After following the advice by @JefferMC  I realized that YES I do have only one IP which is the one from AT&Ts. This IP showed for both tests I did. One over Wifi (Router mode) through the deco and the other one while connected through Ethernet to the gateway. I confirmed I definitely lose some performance by using the AP mode and I will attach the images below if you would like to see the difference.

Now that I know that there's only one NAT on Router mode for me, I think I will stick to this mode for now. I prefer to get the max out of the Decos while I can. I appreciate everyone's  help here a lot! Happy holidays.

I almost forgot this very important and last question. Would the Gateways firewall still protect me even if I'm doing an IP passthrough to the mesh devices on Router mode? What I mean is would the modem's firewall options still work even when I am connected over wifi through these Decos (RM)? I know Tp link offer a homeshield plan for the wifi service but I don't know if I am really going to need it.  

In IP Passthrough the BGW firewall protects anything you connect directly to the BGW, which is usually just wired devices since it's suggested that you disable the wifi on the BGW.

The Deco has its own firewall that protects all wired and wifi devices connected to it.  That's a separate function from the HomeShield plan, which adds on some bells and whistles like parental controls.  They want to sell you a pro version of that with even more crap that I don't think most people need.