I am stuck on configuring the WAN port - or better the port which connects to the Internet. The modem has a DHCP server and therefore anything which connects to it gets an IP.
How can I set up a specific port to act as a WAN port please? NAT masquerading?
Set the modem into the transparent bridge mode and set the wan port of the router according to the information you have from your isp. It is not so smart to have dual nat on the way.