Using a ConnectME -C running Net+OS 7.5 and custom firmware. Firmware is based on the tcpserial example. When the ConnectME is configured to use DHCP, I can connect to it (using TCP) inside the LAN and from outside thru the router (with port forwarded). When the ConnectME is assigned a static address (I've tried several different ones), I can connect to it inside the LAN, but not from outside thru the router. Tracing this in Wireshark reveals that when the outside connection request hits the router's WAN port, the router broadcasts an ARP request for the IP address it is forwarded to. But the ConnectME never responds!
However, change the ConnectME back to DHCP, repeat the test, and the ConnectME responds to the ARP request with it's MAC and the connection succeeds. Likewise, on the LAN, whether the ConnectME has a static or dhcp address, the originating computer issues an ARP request and the ConnectME responds and the connection succeeds.
This happens on three ConnectMEs running the same firmware on three different LANs behind three different routers.