And a simple wireless network card was not an option for each of these LAN-PC's without Wifi ?
Now you are making things so complex.
I think you need to look into "dhcp relay" where you want to capture the DHCP request on the LAN-side (from the PC's) and forward it towards a Microsoft DHCP server somwhere behind the Unify WLAN setup ?
https://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Relay
You must make sure the Mikrotik off course can reach the DHCP-server.
And I wonder how you realized it because so many things can go wrong. Is this setup "bridged" (do the PC's use the same IP-space then across the wireless ?) Or is it some routed LAN-network across the wireless ?