ok i figured it out.
Putting the RUTX09 in bridge mode works like described in the docs and like suggested. I put in the mac address of the WAN interface of the USG, eth0 in my case. The USG gets the IP.
I needed to powercycle the RUTX09 in order to function again after configuring bridge mode. But ok, no problem.
After that, i had TLS handshake issues. This means MTU/MSS issues most of the time. after searching for my options, i found out (after quite some time/trying) the unifi controller was not able to properly set the MSS value from the Web GUI for the WAN interface. It did not work, no matter what value i would set for MSS on the USG WAN interface advanced settings. So i fell back to the CLI. that worked.
i found out that a "mss" value had to be set for "all" interfaces on the usg. the commands below will do just that. I think the controller interface tries something different under the hood, with less success. the maximum value i could set was 1430. 1431 gave problems, 1430 was ok.
i had to configure the USG with the following commands. You can find out the value that works best for you by using the commands and just see where it breaks.
Looking good, i'll test some more.
This website is usefull to display your MSS and MTU settings as a result of this configuration changes.
set firewall options mss-clamp interface-type all
set firewall options mss-clamp mss 1430