Enhanced wifi and wifi password

Both WiFi bands on the gateway are on the same network. You can rename the networks and give them different names and passwords. Set your smart device to connect to the network you want or turn off the WiFi band you don’t want the smart device using. 

2) Our installer said NOT to change the pw as it might interupt the firmware updates/etc....sounds like bunk to me and unfortunately ATT hasn't picked up the trend to have a memorable pw....is there any reason not to change this to something I can remember?

No reason at all.  Of course you can change them.  The only time you lose your personal gateway settings (any settings) is if you do a factory reset by pushing the button on the back of the gateway.

Note, the passwords are "difficult" for a reason, i.e., for additional security to keep someone from trying to hack into your wifi.  Yes, it's a limited range but not necessarily limited to within your house.  Also most devices you use, I would think, have a preferred/default network setting so you only need to chose the wifi by network name (ssid) and your device remembers the corresponding password.  So make your network name something you can recognize in the list of accessible wifi networks but let the device remember its hopefully difficult-to-guess password.