Instagram captions do not support real bold. The workaround is to type the caption with unicode bold characters, which look bold in every field and every device.
Type your caption normally in a converter such as Font Style, copy the result, then paste it into the caption box. Hashtags survive the conversion, but keep them in plain text if you want them to be clickable.
Use bold sparingly: the first line and maybe one call to action. A fully bold caption looks like shouting.
Written by FontStyle - more at fontstyle.io.