You cannot really expect OBi to give support for networking issues that are nothing to do with their product.
DMZ is a facility on most routers but it is not applicable to wireless adaptors, PCs, internet connection sharing, bridging etc so you will search in vain for it on any of those. Also while placing an OBi in a router's DMZ can be useful for diagnostic purposes it is really an absolute last resort to leave it there as a permanent configuration.
Have you configured the OBi IP address settings for DHCP or static?