Werner
I operate the first Airport extreme as a wireless router (DHCP), with some
things attached through the ethernet ports (Sonos base station) I also have a second airport extreme (bridge mode) in the basement where the AppleTV is located.
I have changed the settings on the IB2 box In “IP Settings”, (in “expert” mode ) I activated DHCP and gave the AE base station a static address (2 digits higher than the swisscom TV box).(image below) Then on the AE base station I also engaged DHCP, but with the 10.0.1.0 starting address. (image below) It is working but I get a “double NAT” warning on the AE base station. which is logical.(see images below)
I have ordered a technician because I want to get to the bottom of this problem. I will try to post the outcome and final settings on the forum, if he can do a better job.

