There could be a couple of different things going on here but something could be blocking your adapter from using or scanning the 2.4 GHz band. Can you try:
Disassociating from any network and then opening Chanalyzer (to check if a band steering mechanism is at play)
Check the WiFi adapter driver properties and make sure the 2.4 GHz band is enabled. You can find this under Control Panel > Network and Internet > Network Connections > Right click on adapter > Properties > Configure > Advanced. Different adapters have different properties here, but look for something that indicates 2.4 GHz usage, like this:
Finally, try using an external WiFi adapter like the Edimax EW-7833UAC AC1750 here since it supports 3x3 spatial streams and see if this helps display both bands.
Let us know if that helps!