I should preface saying that I spoke to Mikrotik sales and they said my application was possible with the RB751U-2HnD.

Our application for the RB751U-2HnD is connecting an embedded device to the PoE "gateway" port that will both power the RB751U-2HnD, broadcast UDP and it hosts web pages for its UI. We need the PoE port to look like an ordinary LAN port and be bridged to the other LAN port. We will then use the wireless as a AP to link to Iphones, etc.

First I selected Interfaces > ether1-gateway. Under Master Port, there is a selection for ether2-master-local. When I change it to ether2-master-local and Apply, I get a message that "master port and bandwidth settings are not supported on this port".

Am I missing something? Should I be able to change the gateway port as I need to for my application?

Thanks.

you should add bridge interface and add ether1 and ether2 as ports to that bridge. Or use ether1 just to power router and use ether2 to ether5 to connect different Ethernet networks using switch feature.