If your app is on Play store showing live ads -- you can't use live ads for testing -- add your device ID in code to get test ads from Admob on your real device. Never use live ads during development or testing.
To get real device ID in logcat,
Open any app on your device which shows live ads from Admob: On the connected device, if you have your app downloaded from play store(showing live ads) open that app or else open any other app that shows live Admob ads. Your device should have an internet connection.
Filter the logcat with 'device' as shown below to get test device
Read Admob ad testing on device - device IDs can change for more