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Install software:

opkg update
opkg install aircrack-ng

Test current wifi mode:

iw wlan0 info
Interface wlan0
        ifindex 5
        wdev 0x2
        addr 90:a2:da:f0:06:76
        type managed
        wiphy 0
        channel 8 (2447 MHz), width: 20 MHz, center1: 2447 MHz

Start monitor mode:

airmon-ng start wlan0  1
wlan0           Atheros         ath9k - [phy0]
                                (monitor mode enabled on mon0)

Confirm it under monitor mode:

iwconfig
mon0      IEEE 802.11bgn  Mode:Monitor  Tx-Power=17 dBm
          RTS thr:off   Fragment thr:off
          Power Management:off
iw mon0 info
Interface mon0
        ifindex 6
        wdev 0x3
        addr 90:a2:da:f0:06:76
        type monitor
        wiphy 0

Get BSSID and Channel:

airodump-ng mon0 
BSSID              PWR  Beacons    #Data, #/s  CH  MB   ENC  CIPHER AUTH ESSID
40:70:09:E4:87:90    0       12        0    0   6  54e. WPA2 CCMP   PSK  TG167
F4:6D:04:5D:1B:BC    0       37        0    0   8  54e. WPA2 TKIP   PSK  dd-wr
D8:50:E6:D7:EA:1E    0       43      161   29   8  54e  WPA2 CCMP   PSK  ASUS

Start Capture:

airodump-ng -c 8 --bssid D8:50:E6:D7:EA:1E -w capture mon0

Stop monitor mode:

airmon-ng stop mon0