In this case please try changing the security mode option on the GUI of the modem and also change the Wi-Fi name then after saving changes on the modem interface you can try refreshing the Wi-Fi on your device in which you need to connect to 2.4 network.

After logging into the modem GUI on the home page go to

Gateway > Connection > Wi-Fi > Edit 2.4 GHz

select security mode drop down arrow and select WPAWPA2-PSK (TKIP/AES) (Recommended) option then save.