Yes you can do some changes on your modem settings to connect particular device to your modem 2.4 WIFI network. Please try accessing your modem settings page or GUI

1. ​Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://10.0.0.1 into the address box, and press the Enter key.  The Login page will appear.
2. On the Login page, enter admin in the Username field.
3. Enter password in the Password field then click the Login button.  The Gateway > At a Glance page will appear. 
4. On the left-hand menu, click the Connection link then click the Wi-Fi sub-menu link.  The Gateway > Connection Wi-Fi page will appear.
5. Click the Edit button for the 2.4GHz Frequency Band.  The Gateway > Connection > Wi-Fi > Edit 2.4 GHz page will appear.  
6.  select security mode on the interface click the drop down arrow and select option WPAWPA2-PSK(TKIP/AES) Recommended and make sure the 2.4 network is enabled then check on the device to connect to the 2.4 WIFI network.

You shouldn’t need to but because not all app designers/coders do a decent job, you also need to ensure the phone you’re using is connected to the 2.4ghz network during setup as well. For the most part, a decently coded app would allow the phone on the 5ghz during setup but again, not all app designers want to put the extra effort in