Correct, but leaving him or her alone with this fact isn’t helpful.
Please check out this Community Tip. Most of 521 errors have been solved by following the steps.
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.
Let us know if you still need help when you tried the quick fix ideas. There are also a lot of threads here with more troubleshooting hints.