Sleep on it again one night and think about it in a structured way: I'm a bit embarrassed: The error was due to a faulty DNS server configuration. Many thanks for your help!

@Clive_A What is strange is that it works with the IP address, but not with the dynamic name (DDNS). Approx. 2 seconds after I enter the IP address in the inform URL field, the remote ER-605 is ready...

On the remote router I can login with the device admin account. So it's not managed by the controller. (Thanks so far!)

sorry, no: 192.168.0.100 is only the computer with which I am on the router's web interface.

yep, i tried it now with the discovery utility: it doesnt work neither. I am logged out of the controller with the message: "Note: This Gateway is being managed by Omada Controller 192.168.0.100"....

I activated the ping response on the ER-605 to make sure there was a connection possibility. The IP address is identical between DDNS and WAN IP of site 1: when I use the ping function (System Tools:...