In this case you can try changing security mode for the modem Wi-Fi network and  also try changing Wi-Fi channel for both bands 2.4 and 5G then connect to all your devices and monitor connection for next 24hrs. To change the security mode login to the modem GUI and then go to 

Gateway > Connection > Wi-Fi > Edit Home 2.4 network > select Security mode drop down arrow > select the WPAWPA2-PSK (TKIP/AES) option > save settings

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Channel

After changing Wi-Fi channel as recommended on the link restart the modem then we would request you to monitor the connection for next 24hrs.