If you alaways been asked for 'Sign in' and still requires activation, please try to remove Office license files on a Mac.
Run Microsoft_Office_2019_VL_Serializer and it will automatic activate Office 2019 Microsoft_Office_2019_VL_Serializer.pkg official link.Microsoft_Office_2019_VL_Serializer.pkg in this gist.Manual download and install Microsoft_Office_2019_VL_Serializer.pkg