To see what IPs are blocked type the following:
iptables –L –n
This will output the iptables list and at the bottom you will see the Chain fail2ban-SSH
Chain fail2ban-SSH (1 references)
target prot opt source destination
RETURN all — 0.0.0.0/0 0.0.0.0/0
To remove the customer’s IP from the block list type the following:
iptables -D fail2ban-SSH -s IP -j DROP
Switch the IP with the customer’s public IP address.
Credits: https://help.webcontrolcenter.com/kb/a1023/unblock-ip-from-fail2ban.aspx