Ok! Sounds like web.conf might be the right file for these settings. Let me know if, after adjusting more of the settings in web.conf, you aren't able to get the right configuration. It's also possible to open a support ticket for more troubleshooting.
Also, you might have already seen this, but just FYI: this older post went into a lot of detail on a reverse proxy configuration and addressed some differences in software versions 5 and 6: