Error
Try the suggestions in this Community Tip to help you fix Error 521: Web server is down.
Background
A 521 error happens when we are unable to make a TCP connection to your origin server. Specifically, Cloudflare tried to connect to your origin server on port 80 or 443, but received a connection refused error. This is often caused by security or firewall software and happens if the origin server has directly refused Cloudflare’s proxy request.
Quick Fix Ideas
Run Server Linux and run code bellow
nano cloudflare2.sh
iptables -I INPUT -p tcp -m multiport --dports http,https -s "173.245.48.0/20" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "103.21.244.0/22" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "103.22.200.0/22" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "103.31.4.0/22" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "141.101.64.0/18" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "108.162.192.0/18" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "190.93.240.0/20" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "188.114.96.0/20" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "197.234.240.0/22" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "198.41.128.0/17" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "162.158.0.0/15" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "104.16.0.0/12" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "172.64.0.0/13" -j ACCEPT
iptables -I INPUT -p tcp -m multiport --dports http,https -s "131.0.72.0/22" -j ACCEPT
chmod 777 cloudflare2.sh
./cloudflare2.sh
Then restart server
