For Authentication failed errors, try an unlock captcha for the Gmail account from a computer - I'm new here and can't yet post links but make a general google search google display unlock captcha and you'll find the link. Also, in gmail, allow access to less secure apps (there was a google help center article somewhere about this, google gmail allow access to less secure apps and you'll find it, from the article there is a link directly to the setting) and try adding it again afterwards. Workaround: Use Gmail app if you have the storage for it.
Hope this helps.