You might try different POE injectors. I did not look up that Ubiquity model, but some POE injectors (older models) are only wired for 100mb. You should be able to tell by the LED colors on the switch about the connected speed. Also, this may have been mentioned above, try setting manual radio channels on the 2.4G band. 1, 6, and 11 do not overlap. Each AP on a different channel. Then maybe leave the mesh unit to auto. You can also bind the mesh unit to a single AP if you like.
happy holidays everyone .
UPDATE to the low speed 7664 units.
deployed on a new project.
1 x GS-GWN7001
2 x GS-GWN7701P
4 x GWN7664
speeds do not exceed 50mpbs.
hoping next firmware update will sort this issue out.
What are you channel and bandwidth settings?
Happy Holidays.
Have you tried hooking up just one AP at a time, test each one individually? In an earlier post you said with just one it worked until you added more so wondering if one of the APs may have an issue.