I found it worked out of the box. There is some malarkey with the Hub 3 where once you've put it in modem mode you need to turn it off and then on again, and when it comes on it locks to the first MAC address it finds, so if you had your PC connected when powering on after selecting modem mode it may not then connect properly when swapping that lead for a new router. I don't know if it matters which port you use, but my Deco is connected on the bottom port - suggest you try the following:
turning the hub to modem mode,
turning the hub off,
unplugging all ethernet leads from the hub,
connecting the Deco to the bottom port
and restarting the hub.
That should allow the lead unit to get an internet connection - you may also need to check the Deco operating mode is set to Router in the Advanced settings page of the control app. If all of this still doesn't work its more likely we're overlooking something about getting the hub into modem mode and connecting a router, (rather than faults with either the Deco or the Hub 3) so may be best to start a new thread to benefit from the wisdom of people who may have lost interest in this thread.
In terms of are you missing anything by using the Deco for wifi and the hub as a wired-only router, the answer is a bit, but not a huge amount. In Access point mode you've still got the benefits of mesh and multiple nodes, and you're avoiding the Hub 3's meagre wireless performance. The Decos were designed to manage a lot of devices and are five years or more newer technology than the Hub 3, so I'd recommend using the Decos in router mode if possible, but for most domestic use you probably wouldn't max out the Hub 3's meagre device management capabilities, and at a consolation that gives you three spare ethernet ports to use on the Hub 3. In the event that you choose to keep the Hub 3 in router mode with wireless turned off, you would need to make sure the Deco is running in access point mode, because otherwise you risk peculiar conflicts of having two routers fighting over IP addresses, although it is possible that the Deco is clever enough to recognise when there's a router already active.
When you have a mesh device such as a Deco which is designed to replace your router you should use it as intended (as this will utilise its full capability) and put the hub into modem mode. If you leave the hub in router mode you'll then have two DHCP servers running, something which makes no sense in the typical home network.
I did successfully switch the router to modem mode and I am now happy to stick with that opposes to router mode.
Errrmmm.....I didn't quite follow that?
You are at liberty to use either the Deco M4 or the Hub 3 as a router. If the Hub 3 is in Router Mode, then you must set the Deco to Access Point Mode. If the Deco is the router, then the Hub 3 must be in Modem Mode. I'd recommend using the Deco in Router Mode and the Hub 3 in Modem Mode, but either approach will work.
If you have both Hub 3 and Deco in router mode, then there's a high risk of problems as they will both try to manage connected devices causing a range of interesting but unwelcome problems.