MJF,
I just looked at the datasheet for that antenna, it is a single feed antenna and you are plugging it into a MIMO AP that has three outputs. You should really use an antenna that provides three inputs with this AP, if you do not terminate the other two ports you are likely going to damage the AP.
For troubleshooting mesh connections I use these commands on the CLI frequently:
show ap mesh neighbors ap-name <portal name> names
show ap debug client-table ap-name <portal name>