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I'm not really sure, but looking at the
Draytek product information I think you would connect the cable modem ethernet port to the fourth gigabit LAN port. Power off the modem before you connect as the modem won't know it has to speak to a different device.
No doubt there are configuration settings in the Draytek admin controls (access from a web browser on a computer that is connected to one of the other LAN gigabit ports). I think you are going to need to get out the Draytek manual to read up in detail on the config settings you need in the router.