Just following up with a message from D-Link customer support that may help
somebody (I haven't had the nerve to try it out yet):
"We understand from the email that the modem (Zyxel Prestige 645R-A1) has an
inbuilt router.
To connect a router (DI-714P+) to an existing router (modem with inbuilt
router), please follow the steps below:
Step 1 Do not use the WAN port on the second router(DI-714P+) . Use a
cross-over cable and connect for LAN port to LAN port.
Step 2 You will need to change the LAN IP address of the second router
(DI-714P+) to something other than the default 192.168.0.1 (192.168.0.2
for example).
Step 3 Disable DHCP on the second router. To disable DHCP, go into the
routers configuration > Home > DHCP. Click Disable and then click apply.
Step 4 You computer will not receive a DHCP address from you DHCP server so
you will need to set the IP addresses statically on your computers.
To view the FAQ on how to set your IP address statically please visit:
http://support.dlink.com/faq/view.asp?prod_id=1053
*** If the issue persists, it is best you speak to live technical support.
Should you require further assistance with your D-Link products, please
reply to this message, or call toll free at 877-453-5465. Thank you for
networking with D-Link."