Ok. When a client computer connects to a network (wifi or wired), it sends out a DHCP request packet. This is a broadcast packet on a Layer 2 network. This means that the DHCP server will only receive the packet if the server and client are on the same broadcast domain, as in the same VLAN or subnet. Since you have routers between the clients and the DHCP server I am thinking that they are not on the same subnet. For these cases, network equipment manufacturers have mechanisms to forward some broadcast traffic from one subnet to an IP on different subnet (the ip of your DHCP server, in this instance). Different vendors do it differently, so you'll have to find out how yours is doing it.
Of course, this is based on my assumption that the devices are on different subnets and the traffic is routed.
You should check by connecting a client machine to the same VLAN as it would be on wifi and check if it gets a DHCP response. That will tell you that it's not a WFI issue, but just networking.
If my assumptions are correct and you can't do forwarding (bootp or dhcp), you may want to do DHCP locally, at each site. The routers at each site will most likely have DHCP functionality that you can use.
