If you encounter a "Network Error" or "Server Maintenance" message in the APK, you are unfortunately at the mercy of ASUS's server uptime. The Zenfone 9 utilizes a proprietary handshake with ASUS servers, meaning there is currently no reliable "unofficial" way to bypass the tool if the servers are down.
There were reports of ASUS charging approximately $200 to unlock bootloaders at service centers in some European regions, but this is not a globally available or official consumer service. Why the "Paper" Procedure No Longer Works zenfone 9 unlock bootloader
ASUS pulled the "Unlock Device App" (APK) and shut down the backend servers required to authenticate the unlock. ASUS Official Stance: If you encounter a "Network Error" or "Server