The only info I can find is that in 2014 virtual port was not supported.
no support for Virtual Port
On that model AP you can only use native cell (separate BSSID per AP, i.e. a typical micro-cell deployment using different channels) or you can use Virtual cell (common BSSID across all AP's that have identical radio parameters). I'd recommend virtual cell, but make sure the AP's are on the same channel, have the same MIMO and use the same channel width -otherwise the virtual cell will be broken.