I have set up a DNAT to forward port 80 to an internal server, but it's not working. - DM4 = - I have the DNAT entry as WAN ->HTTP ->DM4 ->HTTP - Is there anything additional I need to do?

The DNAT entry should be:

WanInterface -- HTTP -- DM4 -- HTTP

You also need to add a packet filter rule:

Any - HTTP - DM4 - Accept