I agree that the Fastmail Android or iOS application should work fine with your main login address and password. I have the Fastmail iOS app and it works great, but I don't have an Android device and don't know if the Fastmail Android app works well with your LG phone.
If you are sure you are using the same credentials you use to log in using the web, then you need to file a support request so that Fastmail staff can assist you.
Bill
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