R7500 stuck at network mode b/g on 2.4Ghz band

Using Wifi Explorer on the Mac I can see that my Netgear R7500 is using network Mode b/g on the 2.4Ghz band and is thus limited to a Max rate of 54Mbps - even though the Mode setting in the router configs lists b/g/n - as a result I'm seeing poor wifi performance on that channel. Pretty sure it wasn't always like this. Amy ideas what might be forcing the lower speed ?

Ian