https://support.cloudflare.com/hc/en-us/articles/115000479507#h_30cc332c-8f6e-42d8-9c59-6c1f06650639
This is hard to believe, considering nothing is running there at all → sitemeer.com/#https://54.154.121.59
Or are you saying it is running fine when you have your self-signed certificate, but currently not running at all because you dont have a valid certificate? That would be a completely different issue then and the 521 wouldnt be a surprise of course.
However the issue then is not to fix the 521, but to configure your server properly to accept the certificate. Which, again, brings us back to your vendor 