If it is missing from the main site, check the Ubiquiti Community forums where users often share direct links to archived files hosted on the official dl.ubnt.com servers.
Ubiquiti community members and network administrators often mirror older versions of firmware like v5.5.6. The Ubiquiti Community Forum serves as an excellent resource where users post verified SHA-checksums and direct repositories for safely restoring legacy airMAX devices. Step-by-Step: How to Flash Your Device Ubnt Firmware Xw.v5.5.6 Download
Do NOT turn off the device during the flashing process. The radio will restart automatically. Important Considerations & Safety If it is missing from the main site,