As I understand the “Roboto” font is one of many available in BS5 so I chose it as my base font in theme manager. Then in my custom css .utg class I added the font weight 700.
Should I include it another way ?
You added Roboto weight 300 as your base font, that’s why 700 is not available.
If you want to use different font-weight options, yo’d better include the font on your page, selecting the font-weights you want to use, as explained here: https://fonts.google.com/specimen/Roboto?query=roboto then you will be able to assign custom font-weight values for your custom css classes.