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Do you mean removing AP1 and AP2 and switching out AP1 with AP3 and then AP4? Or should I make AP3 an additional Root AP while keeping AP1? One more hint: Is it maybe a problem that I used 2.4GHz for WDS?

Additional info: After turning off AP2 and then turning it on again 2 minutes later, now also AP2 does not connect to AP1 any more. Can this really be that complicated? It seems that is one of the most important use cases for these access points?

Currently the APs run in standalone mode. Would it make a difference if I put them on Nebula?

I provided the diagnostic information in a private message to Zyxel_Nami. Please let me know once you received this.

All devices use the attached power adapter. No PoE. Collecting the diagnostica info will require time, because I'm currently traveling. I'll keep you posted.

Hi Nami, thank you for your reply. Unfortunately, nothing changed. What I have done: I put AP 4 on the ethernet uplink, physically next to AP 1. I verified that WDS SSID, key, VLAN and profile name are identical. I added the MAC address of AP 1 to AP 4 as you suggested. No WDS connection is established.