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

Yes, Router Mode is what it needs to be for Passthrough.

If you're basing the "double nat" conclusion on a tracert then that's not accurate.  The BGW will show up as the second hop in a tracert but it is not doing NAT for traffic going to the Deco.  It does have some processing to do (remember, this is not a true bridge mode) but that doesn't affect the Deco.  With the BGW's firewall on, it still does not affect anything going to the Deco, just any traffic on devices that you might still connect to the BGW directly.

The BGW210 does not support Wifi6 itself.  AT&T does have the BGW320 that does Wifi6 but because of shortages and other factors, they reserve them for >1Gbps service or when appropriate for the local infrastructure.

You can use IP Passthrough to hook any third party router or mesh system to the BGW210 to give you Wifi6 or even Wifi6E.  I've yet to see anything from the major networking companies that isn't compatible, but that's not to say that some don't work better than other when it comes to wifi.

Thank you so much for your information. I read that IP passthrough has its own issues and I am curious if there's a way to do a True bridge mode. And if so, would I need any specific equipment for it? Example: specific brand of router or modem/router. 

No true bridge.  I agree that IP passthrough is less than desirable.   

If the main purpose is to improve WiFi, and I personally encourage this path, I’d connect up something that can run in access point mode and let the gateway handle routing.  While you’re at it, go for 6E or wait for 7 if you can.      

I was not a fan of IP Passthrough on my BGW210 but because of a need to test using that setup, I switched over to it back in August.  It's been fine, no issues whatsoever.  It's the closest thing you're going to get to bridge mode with any AT&T gateway unless you explore unauthorized methods of bypassing.

I try not to recommend specific brands or models of third party routers because everyone's environment and requirements are different.  What works great for my home might be a dud in your home because of layout, size, materials, what's in the way of wifi signals between the router and where you need wifi, etc.  So I always suggest people read up on routers and wifi mesh systems on known tech sites, find something that meets their budget and buy it from someone with a good return policy.  If it is a dud, you can return it and try something else.

@Phobiatoslowinternet 

I am curious if there's a way to do a True bridge mode. And if so, would I need any specific equipment for it? Example: specific brand of router or modem/router.  

No true Bridge mode.option but there are unsupported "bypass" options that you should do a Google Search on these so called "bypass" methods. So use an Ethernet switch or with the BGW320 style installs you would need a SFP+ ONT substitute for the SFP+ Optical transceiver / BGW320 ONT combination.

Dave

I appreciate all the comments provided by everyone here!

My main interest is to be able to control my network wothout sacrificing speed/performance. I am not super tech savi in this field but I can try if there's any good guide out there.

Also, I got  a 1 gig down and up for internet plan. I don't have any wifi signal issues but since there are some wifi 6 compatible devices, I guess the question is could I use any wifi 6 mesh devices and would they work out of the box with this BGW210-700?

Sure, you could get a Wifi6 (or even Wifi6E) mesh system and they'd support your client devices that have that capability, at least in IP Passthrough mode, regardless of which AT&T gateway you have.  If you used AP mode on the mesh system, leaving the BGW's wifi on, whenever one of the clients got close enough to the BGW and connected to it they would drop back to Wifi5 (AX) mode.

Thank you! Is there any good guides you could possibly share to do this? I recently got the mesh Decos XE75 wifi 6e from TPlink and I would like to set them up to have control of incoming and outcoming data through them without having to get the firewall of the gateway involved.  

That would be IP Passthrough and my first comment at the top of this thread has a great link to a tutorial for the BGW gateways.

If you decide to go AP Mode, then TP-Link has a guide for that HERE.