@Carbon6 

Hiya, are you replacing the Vodafone router with the M4 and plugging the ethernet cable from the ONT directly into the M4?

Or are you going from the Vodafone router into the M4?

Have you kept a digital phone line with Vodafone?

If you are connecting directly to the ONT with the M4, you will need your vodafone login and password, use PPPOE and VLAN 101 (if openreach) and you will lose digital phone line.

If you are connecting the M4 to the Vodafone router then you would want to connect it to a LAN port on the router not a WAN port.

Just for clarity.

  @Carbon6 

Hi, 
Please turn off the QoS feature in the Deco app > More > QoS page and check if it helps. If the same phenomenon persists, please perform the tests below and share the results with us.
  1) Please turn off the satellite Deco and use only the main Deco.

  2) Test 1: Connect a PC/laptop to the WAN port on the wall directly. If it has an internet connection, go to speedtest.net to test the speed and record the result (download speed 1). Then, refer to the instructions below to check the wired link speed on your PC/laptop(link speed 1).
How to check the associated/link speed on a wired client

  3) Test 2: Connect the main Deco to the WAN port on the wall, and connect the PC/laptop to the main Deco with an Ethernet cable. Go to speedtest.net to test the speed and record the result (download speed 2). Then check the wired link speed on the PC/laptop(link speed 2).

  4) Test 3: Connect the PC/laptop to the main Deco via Wi-Fi. Go to speedtest.net to test the speed and record the result (download speed 3). Please also check the wireless link speed on the PC/laptop(link speed 3).
How to check the associated/link speed on a wireless client

 5) Please send us the above download speed and link speed test results for comparison and analysis of the slow speed issue.

Best Regards