There is one thing need to be noted:
Never operate the antenna when the router is powered on, which could damage your radio. I am not sure if this is the case. As long as the wifi works fine, i.e. send and receive data well in a meter like 5 or 10 meters, it means the radio is still fine.
Antenna:
Both antenna works in the same node. If you set as 11b, then both are 11b.
Power settings:
check out dBm to Miliwatt here dBm to mW conversion calculator
0dBm is 1 mW
20dBm is 100mW
This is only for TX, i.e. the power the router ouput. It doesn’t affect RX, which is the key factor for the router scan signal.
USB WiFi dongle
There are some usb wifi dongles e.g. those with 3070 and 8187 chips works with router and you can try that.
Summary, use a high gain antenna and you should be able to scan more wifi.