Built-in. Ethernet

Thank you! Is this something I can do or has to be done by ATT&T technician? 

IT is my career so I can do this myself after AT&T tech installs the fiber connection. I don't know your tech skills and the wiring, equipment etc so I can't say for sure if you can/can't do it yourself. It should be as easy as plug n play.

I did exactly this at the work place a few months ago. AT&T tech installed fiber connection and I "split" the fiber connection to 100+ computers at the office over ethernet connection.

It takes literally zero technical skills to do this.  If you can plug cables in you can easily connect the rooms to the AT&T gateway.

You'll need to decide how many rooms need to have an Ethernet connection.  Then buy a gigabit switch that has at least that many ports + 1 for the gateway connection.

TP-Link is one of my favorite switch makers.  Here's a LINK to various models from 5 to 48 ports. 

Then you need some short Ethernet cables to go from the switch to the AT&T gateway and from the switch to wherever the home's Ethernet wiring all comes together.  I presume you have a closet or some space where they all meet and hopefully that's also where your AT&T gateway is located.  Get CAT5e or CAT6 Ethernet "patch cables" which are shorter cables designed exactly for this purpose.

If you want to post some pictures of the place where all the cables come together I can give you more specific instructions. 

But like I said, it's a matter of just plugging the power cable in for the switch and plugging the Ethernet cables into the switch.  You can do this!