Tuan,
In case it's not clear:
Antenna port R1-A, R1-A, and R1-A connect to radio-1
Antenna port R2-A, R2-A, and R2-A connect to radio-2
You've stated that the antenna you are connecting is a 2.4GHz model (does not support 5GHz)
and when connected to the R1-A port it works. This tells me that radio-1 is configured as a 2.4GHz radio, which means that radio-2 is configured for 5GHz operation.
So when you connect the 2.4GHz antenna into any of the R2 ports, I would not expect any signal. When you do this, you are connecting an antenna that only supports 2.4GHz to a radio that is operating at 5GHz.
You either need to use antennas that support both 2.4 and 5GHz or you need to use separate 5GHz antennas to connect to the R2 ports.