Chains 0, 1, and 2 (Ch0, Ch1 & Ch2) are antennae on a single radio. If your radio is 2x2 MiMo then you have two antennae (Ch0 and Ch1).
TX/RX Signal Strength = Ch0+Ch1+Ch2
TX/RX Signal Strength Ch0 = Signal from antenna connected to chain 0
TX/RX Signal Strength Ch1 = Signal from antenna connected to chain 1
TX/RX Signal Strength Ch2 = Signal from antenna connected to chain 2
In the world of decibel, -20dBm + (-20dBm) = -17dBm
The values displayed in the picture are TX values only. This is the situation when you are connecting to non ROS system, the router can only display the TX signal strength from you radio.
The Ch2 field is empty because your radio is 2x2 MiMo with only two chains.