IP Block Issue

[tetrahost]

One of client informed me that, he is unable to access their site hosted in my server. I then accessed their PC using anydesk, their IP was in firewall whitelist still they were unable to access. I then stopped the firewall using command line and it started working, then started the firewall and it stopped working again. I then stopped CSF and started the firewall from WHM menu and then it was working.

So not sure what is causing the issue. your help will be appreciated