Please excuse me for not noticing your IT Director badge on your lapel when you posted your second post at this forum. Most people who come here with wireless questions are often novices who bought an Alexa device and are often uninformed about how wi-fi works at elementary levels, etc.
I have this to say about Echo support line: If you have a simple question that the first tier support can answer and say, "turn it off and turn it back on," fine, otherwise, they should have shifted you to the 2nd tier of support, and if second didn't have a clue, they should have shifted you higher. If they didn't, you should have requested you be transferred to the next tier. To tell you to mess with the security settings is ridiculous. Unless your unifi system is quite different than most home systems, the settings I mentioned are what they should be set to.
If your Dots and Echo worked with your previous wi-fi fine, my background as an electronic tech suggests to me that the Echo was fine, but its interaction with the new system is not. So, to keep swapping out Echos (even the first one) was kind of a clueless move on Amazon's part. Obviously, the Echo was a victim of the problem, not the cause of it.
UNIFI appears to be an "industrial strength" wireless system for motels, hotels and even farms, etc. If that was my home system, I would be talking this over with Unifi tech support, not Amazon's.