How do I edit the Wifi Mode (2.4 and 5 GHz Wifi channel settings) rather than automatic

"If you don't see the 2.4 and 5 GHz WiFi channel settings (i.e., WiFi Mode, Channel Selection and Channel Mode), it means that these settings are being managed automatically to help optimize your home network and provide the best performance possible."

"Wi-Fi Mode, Security Mode, Channel Selection, Channel Mode, and Channel Bandwidth are being managed automatically to help optimize your home Wi-Fi network and improve Wi-Fi coverage."

The first message is what I get on the Xfinity website to change my Wifi mode and the second is via the router settings.  I have a device (Nintendo 3DS) that I am trying to connect to the internet.  The issue is that the 802.11 mode needs to be changed to connect to the internet because the default (802.11 g/n/ax) doesn't work but the option to change it is greyed out so I can't do anything.  802.11 b/g/n is what I want to switch to. How do I manage these settings manually?