Expanding channels on AR7700 with PX4

Hi! I am working on a UAV with AR7700 and Pixhawk 1 using the PX4 flight stack. We have been trying to expand the channels on the AR7700 to greater than 8 by using the SRXL or remote receiver ports, however we have been unsuccessful at actually getting this to work. Would anyone be able to provide any information about how to expand the channels with the AR7700? Been searching for awhile and have not really found a concrete answer. Thank you!