I’m not sure which IP address you’re talking about. If it’s the public IP address, that doesn’t come to play at the server end. The general rule of thumb is that 5xx errors are due to a connection error between the server and Cloudflare.
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. This often requires assistance from the web host.