If an AP fails, use. There are differences between APs and this was the current one that worked for:

https://community.ui.com/releases/UniFi-Access-Point-6-5-66/892a2c14-cfd5-4351-986e-607c505f14c3

https://help.ui.com/hc/en-us/articles/360043360253

tftp
connect 192.168.1.34
binary
rexmt 1
timeout 60
put BZ.qca956x_6.5.66+14789.230802.0330.bin

Remember though, that APs can fail due to fallback IP address conflicts and that won't help failed adoptions.