If you want to configure sta using ardio0, you need to disable the original sta. Pls check the highlighted config below.
Then you need to add the following
config wifi-iface ‘sta1’
option device ‘radio0’
option network ‘wwan’
option mode ‘sta’
option ssid ‘Airtel-E5573-5656’
option encryption ‘psk2’
option key ‘xxxxxxxxx’
option ifname ‘wlan-sta’
option channel ‘xx’
About channel, you may not need this. But if sta cannot be connected, pls add channel information.