I’m having the same issue as the OP, and I went to look at my 2.4 settings on my Wi-Fi, and my Xfinity router does not allow for me to change my settings - it’s the second one listed in the link you gave and states that advanced controls are not accessible.
More of the ISP are preventing user configuration of their routers. Two options are available to you.
1. Call the ISP (Xfinity), and have them make the change for you. Don't let them tell you they can't. Call back if you get an unhelpful/unresponsive tech support agent until you get one that will change it for you. They can prevent you from making the changes yourself, but they cannot prevent you from enabling/using legacy protocols on your network. Tell them to "change the wireless mode to b/g/n on your 2.4ghz band".
2. Purchase your own router. Then place the ISP router in "bridge mode". With this configuration, the ISP router will act only as a modem (for internet connection and access), and your new router will act as the "router" and you will be able to configure as you see fit for all your devices on your network.
Thank you! I’ll give them a call tomorrow
