Some attackers may record real server IP history, and the exposed IPs allow them to bypass Anti-DDoS Pro and directly attack your real server. In this case, you are recommended to change the actual real server IP.
You can refer to this document before changing the real server IP to check the risk factors and prevent the new IP from disclosure.
Check all the DNS resolution records of the attacked real server IP, including resolution records of sub-domain names, MX (mail exchanger) records of mail servers, and NS (name server) records. Make sure that all these records are configured to the protected IP so that the DNS will not resolve to the new real server IP.
Check your real server for potential trojans, backdoors, and other hidden risks.