I know that you have tried adding the bolded font to a font family, but it hasn't worked. You can try one of the ways below. It will work even if you do import a TTF font.
One way you can do it is in strings.xml
You can create a new string and then add the b to bold it like this:
<string name="my_string"><b> My String</b></string>
You will need to create one for each string that you want to bold.
Then in your layout file, you can set the android:text
to one of the strings from strings.xml and you can set the android:fontFamily
to the TTF font.
You can also bold it programmatically using the SpannableString in Java. Check this article to see how to bold programmatically.
Hope it helps!