Login to the modem GUI or modem web interface and on the interface select wireless 2.4Ghz option on the top menu > AP isolation > you can enable or disable by checking into the box.

Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://192.168.0.1 into the address box, and press the Enter key.  The Login page will appear.
On the Login page, enter admin in the User Name field.
Enter password or a customized password in the Password field.
Click the Apply button.  The System Basic Setup window will appear.