The E4 only has 10/100 ports IIRC and so will never get above 100Mbps.
I would advise starting again and resetting the Deco and following this sequence.
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Disconnect all cables from your new router (or Mesh first unit) and unplug it. Put the VM Hub into modem mode (see VM website for “how to” details) 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. Then, set up your own router/Mesh with the WAN port set to be in DHCP & Nat mode, it may be pre-set that way anyway. Make sure it’s fully initialised (leave 5 min) and then put in the WAN cable to the VM hub. Now power up the VM hub and you should get a connection. Use the recommended App to connect other Mesh units to the first one. This order only needs to be done the first time you connect the router to the VM hub.
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John
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I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.