Hi Gin Gin,
The 5GHz radio would be down if it wasn't configured to transmit an SSID. If you have done this and the 5GHz radio still won't come up then I would recommend doing a factory reset using the button on the underside of the AP and then upgrading to the lastest version of Unleashed (if you aren't already).
Regarding the model specific control, I've just checked on my Unleashed R310 AP and can select any model. This may indicate that something isn't happy with your AP and you need to run through what I've suggested above.