Hello,
And thanks @phpexpert21 for stepping in so quickly.
Indeed the theme, like most, allows to show either one or the other. But actually, when a logo is set, the title is also still included in a hidden H1 tag (for SEO purpose).
You can un-hide it easily with just CSS. To do so add the following in Customize > Additional CSS:
.site-title { display: block!important; }
You may add additional CSS rules to change the position and margins if needed.
Alternatively, you can also make even further changes directly in header.php. It is strongly recommended to create a child theme (see: https://codex.www.ads-software.com/Child_Themes ) instead of editing a file directly from the theme. Copy header.php from the theme into the child and you can then edit this copy as you see fit.