I’ve heard about DMZ before but I never knew what it was. That will probably not be an option unfortunately. While I don’t know what router is currently used by the other residents I assume it will be either a FritzBox (which allow some configuration but are mostly idiot proof routers that are very popular here in germany) or a locked down router by the ISP. On neither case will I be able to configure a DMZ.

Regarding the WAN port, I was planning to use the stock RouterOS from MikroTik but I believe that the router can be configured this way already without OpenWRT.