5268AC in Bridge Mode

Sigh. I signed up for AT&T Fiber, couldn't find any way to ask pre-sales questions, and hoped that, in 2018, I wouldn't end up with hardware that couldn't be fit into my home LAN. I fear I'm about to be disappointed.

Here's my situation: I am switching over from Comcast cable. I have a Cisco RV320 Gigabit Router/Firewall which I would like to continue to use (it has a simple gigabit WAN connection for the upstream connection). My network uses a standalone Raspberry Pi to provide DHCP and local DNS services. This all has worked fine with Comcast, for years.

Unfortunately, the 5268AC hardware I was given -- I wasn't given the option to specify what kind of hardware would be installed -- doesn't want to play nice. Among other things, I cannot figure out how to put it into bridge mode, and, from what I've read elsewhere, that's because it doesn't support a bridge mode. Which is insanely bizarre, IMHO, in 2018. The closest I can supposedly come to what I need is to put the 5268AC into "DMZ+" mode, and mark the Cisco router as being in the DMZ. Only I can't figure out how to do that.

In addition to that problem, it doesn't look like I can turn off the DHCP service in the 5268AC. Which kinda makes my local DHCP/DNS service impossible to use. Unless there's some way to "connect" the 5268AC DHCP service to my pre-existing DHCP service running on the Pi?

Either way, I am extremely disappointed that the package offered by AT&T gave me no information about this, did not make it easy to get a pre-sales question answered, and cannot be configured to meet my needs. Worse come to worst, I guess I'll cancel the AT&T Fiber service, and wait for them to change their ways re: hardware.

I am willing, BTW, to buy my own 3rd party modem to hook into the AT&T Fiber system. One which isn't built to be brain-dead. Are such available? If so, what do people recommend?