To check on this could you please help us with the model number of the AT&T Netgear mobile device.

The Arris Surfboard SBG10 

The Neatgear MR6110

To connect SBG10 with the Net Gear router, I suggest enabling the bridge mode on SBG10 to avoid Wi-Fi interference.

http://arris.force.com/consumers/articles/General_FAQs/SBG10-Bridge-Mode-Setup

After enabling bridge mode, connect the modem to the router through an Ethernet cable.

Hello I have been trying to open the modem GUI from several computers but it won’t open up.

We understand that you are trying to access the GUI from different devices, still unable to login. 

Let's try with a Reset and if still not able to access do update us with the error message so it will be easy to understand the issue.

Check the below link to do a Reset: 

http://arris.force.com/consumers/articles/General_FAQs/SBG10-Factory-Reset