Set it up in this sequence...
With Deco's, you have to do the initial setup of the first Deco unit whilst connected to the hub when it is still in router mode. So wire the Deco into the Hub to get it online and registered Via the App, Then in the Deco settings set the WAN port set to be in Automatic IP i.e. DHCP (& Nat) mode, it may be pre-set that way anyway. Then, disconnect all cables from the Deco and switch it off and unplug it.
Then, put the VM Hub into modem mode
and wait for the base light to turn magenta/purple (on a Hub3 - but not on a Hub4!). Once it’s in modem mode, the VM hub “MUST” then be powered off.
Switch your Deco back on and then put in the ethernet cable from its WAN port to the VM hub (any port will do) and make sure it’s fully initialised (leave 5 min)
NOW… power up the VM hub and wait 5-10 minutes for The Hub to initialise and and you should get a connection.
Use the recommended App to connect any other Deco units to the first one (can take a few minutes to optimise positions) .
This order only needs to be done the first time you connect the router to the VM hub. You might also set the SSID and key/password on the new router/Mesh to match that on the HUB so when you swap over, all your wireless devices should just connect automatically.
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired, 1 WiFi, 1 on PA) Hub5 in modem mode with Apple Airport Extreme Router +2 Airport Express's. On 250Mbps, Talk Anytime Phone, x2 Mobile SIM only iPhones.