Set NGV599 to a different IP address

I don't think you can.  Can't you temporarily access the NVG599 directly (192.168.1.254), for example with ethernet, turn off the 5G wifi, and reconnect your setup?

I see there is a way to change the IP. It is obvs 192.168.1.254.

You are saying it cannot be: 192.168.5.254 and connect back to the ATT's servers?

Por quoi, mon ami?

I said 192.168.1.254 is the IP to access the gateway and as far as I know cannot be changed.  I interpreted your original post as you have another router behind the gateway blocking your access to 192.168.1.254 to get to the gateway and all you wanted to do is turn off the 5G wifi in the gateway.   So I suggest temporarily directly connecting to the gateway with your router disconnected to directly access the gateway's wifi settings.

192.168.1.254 is the way you "talk" to the gateway.  The att servers are not involved.

You may indeed change the LAN-side IP address of your gateway, but of course, you'll also want to change the subnet, etc. to go with that.

I think I understand that you're trying to do this to resolve the conflict with clients behind your router not being able to see the gateway because the gateway and your router are using the same LAN-side address space.  I am running my gateway at 192.168.2.254 for this very reason.

I stand corrected.

Would this be done, in the case of the NVG599, in Home Networks -> Subnets & DHCP -> Private LAN Subnet settings?  That's where I see the Device IPv4 Address (192.168.1.254) and subnet mask (255.255.255.0) in text entry boxes that allow modification.

I can't confirm exactly (without looking at the gateway pages), but that sounds right.