@Hank21 

I went and did some additional testing and in my opinion the issue only got weirder:

- Swapped both EAP225 WALL. Problem remaind at the location, so it's not related to the device or its provisioning

- Checked switch port profiles: they're all set to "All"

- Changed port for the affected location with an AP that's working fine: problem remains the same, at the same location

- Forgot switch, re-adopted, checked all settings: problem remains

- Double checked cable: cable is fine; speed when connected to the ethernet ports of the AP at problem location remains great

- Disconnected an AP from the switch to rule out power-problems (load too high with 5 AP's?): problem remains

- Connected AP from problem location to the switch using a fresh very short cable (1m) that is known good: problem remains (this doesn't make sense, does it?)

It's like the controller had issues with a 5th AP? I think we can assume by testing:

- it's not the AP

- it's not the cable

- it's not the switch port

- it's not the switch

Other remarks:

- only internet access is problematic when connected to WiFi of the problem AP, navigating the controller works ok (maybe because bandwidth requirement is low)

- internet access works but is extremely slow when connected to WiFi from the problem AP

I'm not sure what else to do now. Reset the Omada controller and start from scratch?

  @Hank21 

This is a flat network, no VLANs. All Omada gear has a dhcp reservation in the ISP gateway router. It has also worked fine without any changes to the setup for the last 2 years.

There's only one WiFi network, no guest networks or client isolation. Everything is linked up using CAT6 cable, full copper.

What I was thinking of doing:

- configure the 5th AP as a stand-alone, not connected to Omada, using the same SSID and password. This should confirm it's Omada/config related.

- completely reset the Omada controller and start from scratch

I'll report back with my finding. Thank you for your assistance.