OpenWrt For Ubiquiti Powerbeam-M5-400

I purchased PowerBeam M5-400.
I had a problem creating an openwrt image following the instructions at https://openwrt.org/toh/ubiquiti/powerbeam.

I can't downgrade to XW AirOS 5.6.15 from the web interface.

I tried "tftp recovery mode".
However, for versions below 6.1.4, i.e. versions prior to January 2018, the following error appears after "tftp recovery mode":

Looking at the ssh log,

New (329231) 5.6.15, required (393476) 6.1.4.

The purchased PowerBeam was shipped in May 2019.

Is there no way?