@MR.S It looks like we found the root-cause: the ISP router. We did a factory reset of this router and configured everything from scratch. We are now working on bringing the bench back to its...

@MR.S Just connected the ER605 with WAN-DHCP to the ISP router. Meaning it receives an IP address in the subnet of the LAN-side of the ISP-router. Everything is working as expected; including DNS and...

@MR.S See my previous message: no - the main router towards the Internet is a Box12 (from ISP-KPN).

@MR.S Yes - only problems with the ER605 routers Sure - will make a drawing later today - but for now: Internet === WAN - Box12 (ISP) - LAN === LAN - Omada switch - LAN === WAN - Omada routers - LAN...

@MR.S All ER605 routers are switched off (but still connected). Then only one is powered-on. For this customer, I have arranged remote powercycles All tests are done with this single router (combined...

@MR.S My guess its a different (real!) issue. And with the right problem description. I'm running a single ER605 now and still the same issue. On the WAN-side I'm using DNS-servers from Cloudflare....