Need my 2.4ghz band on b/g/n standard

I was able to split my dual band Wi-Fi into separate 2.4GHz and 5GHz bands through the XFINITY App.  But I also need the 2.4GHz band to operate on b/g/n.  I was on chat support for a long time and the guy kept promising he could Help me but in the end he was useless.

So I need to get this done somehow.  It’s a common problem for older devices that Only work on the “b” protocol.