Channel selection availability is dependent on regulatory domain selection. _________________ "The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep." - Robert Frost
"I am one of the noticeable ones - notice me" - Dale Frances McKenzie Bozzio
I was trying to say these WRT3200ACM are hard-coded from the factory to a specific region, at least transmit power.
But, you can still try playing with dd-wrt webui wireless settings. See advanced checkbox in the first interface wlan0.
There should be a dropdown selection for regulatory domain. Try save, reboot then verify available channels. YMMV.
More info is here also over here, but honestly much better advice is to disable all radios then external access points.
Maybe access points such as CF-E380AC or CF-E385AC, or completely different model router R7800, XR450, XR500.