EX8000 5ghz channel selection for connection to router

I use a R8500 router with Smart Connect selected for 5ghz. I use the EX8000 as a  (expensive) wireless switch (meaning I have no wireless clients for the EX8000, only ethernet conntected devices)--both wireless radios are disabled. This leads to a message on the EX8000 to the effect that I should select a channel above 100; the connection to the router uses the lower band (e.g., channel 36). If I turn on the 5ghz radio on the EX8000, the connection to router will use the high band (e.g., channel 153) and the wireless extension will use the lower band (e.g., channel 36). This is as indicated on the status page. The difference is that with the EX8000 5ghz radio on I get 50% better internet speeds (350mbps versus 230mbps).

This looks like a firmware error, since both the low and hign band are available for selection by the EX8000. Is there a way to force selction and still disable the EX8000 5ghz radio?