If you use it as AP, it defeats the purpose of getting it. That router doesn't support vlan tagging out of the box which is required by unifi and maxis fiber.

If you are plugging it into router it function as AP. The root cause of poor performance is due to the maxis router. If you plug it into BTU(connected to fiber cable), you are using it as router. Making it function as router requires vlan tagging as I had mentioned earlier.