What's the type of the Ethernet cable? I think it's better for you to use cat 5e or above Ethernet cable. And the Ethernet cable should be less than 60 meter.

EAP330 need 802.3 at PoE power, the max power consumption is up to 30w. If you use 802.3 af PoE (the max power consumption is just 15w) to power the EAP330, it will cause the insufficient power supply of EAP330 and some wireless connection issue.

Wireless connection is susceptible to environment interference, it may cause the unstable connection problem and SSID disappearance problem. You can scan the wireless environment by tools, such as inSSIDer or wifi analyzer, and do the wireless optimization according to the scanning result.

1.  Set the channel width of each EAP to 20MHz to reduce the wireless interference.

2. Set the channel of these EAP to 1/6/11 channel respectively in 2.4G radio and set different channel (36/40/44/48) in 5G radio. It will reduce the wireless interference between each EAP.

3. What’s the distance between each EAP? If the distance is less than 15m, you should lower down the TX power of EAP.

Hi, thank you for response.

Ethernet cable is cat6, 5m long (also tried 50cm cat6). All powered by TP-Link switch, according to specification Standard: 802.3af, PoE power: 55W.

Only one AP installed. All testing done in 5GHz band.

Channel change does't help.

Hi, what version you are using? (EU) or (US)

Try these:

1. they just released a new firmware for EAP330 for new controller V3.x.  You'd better upgrade the APs first.

https://www.tp-link.com/en/download/EAP330.html#Firmware

2. Disable Airtime Fairness. This is a good feature theoretically but the effect varies from different clients.

3. Mesh doesn't affect EAP330. 

4. Ensure you didn't change the wireless qos settings. (In fact, I don't know how to change the parameters on that page, too complicated)

5. You mentioned you can get 500/50, that means you are using 11ac clients. I suggest you set up a static 5GHz channel on EAP330.

If you got multiple EAP330, you should avoid using overlap channels. You can use E.g ch36, ch136.

All powered by TP-Link switch, according to specification Standard: 802.3af, PoE power: 55W. Only one AP installed. All testing done in 5GHz band.

I don't suggest you supply power for EAP330 with 802.3 af PoE. EAP330 need 802.3 at PoE power, the max power consumption is up to 30w. If you use 802.3 af PoE (the max power consumption is just 15w) to power the EAP330, it will cause the insufficient power supply of EAP330 and some wireless connection issue.