You said the AT&T gateway is 'normally' at 192.168.1.254.
You scanned the network, what 'was' the AT&T router at? Was it default or was it also in the 76.xx range?

Where did you take your port scan from? Was it wireless on the AT&T WAP built into the router? Or a wired connection from somewhere?

I'm assuming you looked and there are no additional switches or extenders anywhere, since you did not mention that. NVR is wired directly to the AT&T router then.

Assuming you got all the details right, I would conclude the router has been grossly misconfigured. After you get the passwords for everything, I would factory reset the AT&T gateway/router and go from there (start from scratch). Should be a small reset button somewhere you hold for a minute or two.