@CasperS I think it depends on how much you want to keep your existing router and whether your existing router has features on it that you make use of?

Unless you have a good reason to use your existing router as a router then I would place it into bridge/modem mode (some routers name these modes differently so you may need to ask the manufacturer). This way the existing router just terminates the physical connection and passes the line onto the Deco. So you would have your coax into the existing router then plug an RJ-45 network cable into your Deco from your existing router.

You miss out on a fair amount of features if you just use access points mode. e.g. you can't use the parental controls/port forwarding/anti-virus etc.