I haven’t used a debian based distro in almost a decade, so I’m probably not the best help, but it looks like everything is there in the repo you link.
If I try the first link posted, I get " 503 Service Unavailable". As far as I know, that error is in principle for a temporary situation, where a service is down for maintenance, or just too many requests.
And honestly, knowing how to download a package from a 3rd-party should be explained in the package manager (or its docs), not in every repository… It’s even less something darktable should explain (if only because there are just too many distros to make that viable)