However, we’ve linked our custom domain to netlify and that loads just fine at https://weareimd.be
Because we’re now creating a CNAME record for www. that needs to point to our netfliy.app domain, I need to figure out why that default URL is not loading.
Though you assert you have done so, you have not actually pointed your DNS at Netlify:
$ host weareimd.be
weareimd.be has address 35.214.214.223
I don’t know whose IP that is, but it is not ours.
HOWEVER - Netlify WOULD show those same contents at your custom domain had you completed the DNS configuration correctly, so you should troubleshoot that using the link in the 404 message about the support guide - that’s your next steps for debugging the bad deploy here (probably a good idea to do it before you fix the DNS migration, which we will also be happy to advise on if you tell us how you’ve configured things, where, in detail).