I have tested this and so far it works!
I had read about this somewhere but couldn't believe it could be so easy. Especially since Swisscom Service told me I had to input the IP address of the Airport (couldn't find that)
For anyone reading or searching - this is what you do.
Put the Apple Airport Extreme in Bridge mode - that's it.
On the IB2 side, be sure Wifi is turned off, but allow it to assign addresses. In my case I removed the static address I had assigned to the Airport base station. (see images in post from earlier)
Restart everything.
