That’s odd, the link works for me but the wiki was very slow earlier. From what I’ve read Ubiquiti have made it harder to flash new hardware, but even the new ax APs are supported by OpenWRT. There is a commit with some info - it seems there is a way to disable signature verification [0].

[0] - https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=fb...