To setup your custom domain complete the following steps.  We will use the example of https://mygallery.mybrand.co.uk: 

• Enter your chosen domain - this domain must be a domain that you control.  For example if your main web site is www.mybrand.co.uk then enter mygallery.mybrand.co.uk as the custom domain
• LettsArt will check this domain, if ok, then proceed to next step 
• Configure your domain (or get your network administrator to configure your domain), by adding a CNAME record.  The CNAME record should consist of your hostname (mygallery) pointing to cname.vercel-dns.com
• Domain changes may take an hour to update across the internet.  Then enter https://mygallery.mybrand.co.uk and you should see your gallery.

It is essential that you complete each of these steps in order for your custom domain name to effectively apply to your gallery. Otherwise that url will not lead internet users to your gallery.