AmiTCP does not support DHCP as standard. You will need to assign a static IP to the Amiga (as well as DNS information, gateway, subnet mask, etc.) . Just put it outside the DHCP pool range of your router and you should be okay.
If you need DHCP, then you should use the Miami demo. It's not as fast as AmiTCP, and requires more resources (and MUI), but it supports DHCP and is easy to configure.