Sorry, I can't say about that particular modem. My experience with most modem/router units directly from the ISP is that they are restricted in what they can do. I usually recommend people not use rental ISP equipment at all and buy their own modem. Netgear makes a great basic modem for only $60 ( http://www.bestbuy.com/site/netgear-docsis-3-0-cable-modem-black/8860013.p?skuId=8860013 ). If you're paying $10/month to rent the AT&T modem, the Netgear will pay for itself in seven months ( but you probably already know this ).
I don't think you're using "bridge mode" correctly. That's most commonly used to run a virtual hardline to wired devices in another part of your house ( plug your Xbox/PlayStation and cable box into the bridge, then that connects wirelessly to your main router on a sort of dedicated link ). If you simply want to disable any wireless feature on it, I'm willing to bet you can. With Comcast modems that usually involves a call into customer service so they can do it remotely. But again, you're still paying the monthly rental fee. So getting your own modem is usually the better way to go.
I usually recommend you get a separate modem and wireless router rather than a combo unit. While this might be a bit more expensive up front, it can save costs in the future as you can replace one unit or the other if need arises ( if you want to upgrade your wireless performance, you can still use the modem; if you need a different modem for a new ISP, you can still use the router ).
If you need something in actual bridge mode, this is how you'd do it with the Velop. Get your own modem and at least a 2-node Velop pack. The first Velop node plugs right into the modem. Setup your other node next to the devices that need a wired connection ( most often this is a TV, cable box, Blu-ray player, or game console ). Connect the second Velop node to the wireless, then you can plug your other devices into its network ports. If you have more than two devices to plug in, you'll need a switch to get them all through the Velop ( something like this http://www.bestbuy.com/site/netgear-200-series-unmanaged-soho-5-port-10-100-1000-gigabit-switch-white/7902325.p?skuId=7902325 ).