Certain Devices Don't Have Internet

[dewei_lin]

Hello, I have four r510 access points in my house. Since a couple of days ago, the WiFi pretty much just stop working for my phone, the Amazon Alexa devices, it's either completely no internet, or there's a lag of 30 seconds or a minute until my phone/Alexa devices would respond. Also, note that my phone would have a pretty normal connection right after I connect the WiFi (I switch the WiFi off on my phone then switch it back on), and after about 10 seconds or so the WiFi on my phone would when back to no internet or ultra slow. My laptop, however, is doing completely fine, with no lag, good speed. I have turned off the entire system a couple of times and did a factory reset. I reconnect the cables between the controller and the access points but none of those solve the problem. After I tried all the methods I mentioned to fix the issue, I still couldn't think of any cause of this, so any help is appreciated!