react-native-firebase AdMob rewarded not showing

I tried using react-native-firebase module AdMob feature from here: https://rnfirebase.io/docs/v5.x.x/admob/reference/RewardedVideo

When I tried implementing it, it gets successfully initialized using firebase.admob().initialize(), after that, I tried showing a rewarded video using this code but nothing prints in the console and nothing shows up. Is there something I missed?

const advert = firebase.admob().rewarded('ca-app-unit-id')

const AdRequest = firebase.admob.AdRequest
const request = new AdRequest()

// Load the advert with our AdRequest
advert.loadAd(request.build())

advert.on('onAdLoaded', () => {
   console.log('Advert ready to show.')
})

advert.show()