I've always used a separate router from the modem, and at this point the AT&T modem/router doesn't have enough ports and would require fussing with. Which is why I stuck with my existing router. A NAS device is way more tech than I need and I'm not interested in learning how to do the RasperryPi, Linux, etc. Just a simple setup with a networked external drive to backup the computer. But the networked part of it would probably be easier with a newer device and friendly software.

If it would speed things a little to take the router out of the sequence I might consider it, but would have to probably add another of LAN switch because the AT&T device doesn't have as many LAN ports. Originally I wired the phones in the house, then switched those over to data ports. Things like the streaming devices to the televisions are all connected via LAN for faster and more reliable streaming. I just looked, the AT&T modem does have a USB port.