I connected the fonts in my project, in the development environment a new Google fonts are displayed
and on my device (API 25) the fonts remain standard
<TextView android:id="@+id/message" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_gravity="center_vertical|fill_vertical" android:layout_marginStart="@dimen/activity_horizontal_margin" android:layout_marginLeft="@dimen/activity_horizontal_margin" android:layout_marginTop="@dimen/activity_vertical_margin" android:layout_weight="1" android:text="@string/title_home" android:textSize="16sp" app:fontFamily="@font/orbitron_black" app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintTop_toTopOf="parent" /> 

app:fontFamilytoandroid:fontFamily. And show the contents ofbuild.gradle- maybe your library is not connected at all. - Yuriy SPb ♦ pmandroid.support.v7.widget.AppCompatTextViewand returning theapp:fontFamily- YuriSPb ♦ 5:58 pm