Are you connecting to the 5 GHz network? Depending on the type of computer, one or two Virgin Broadband WiFi networks should be shown in your list of networks and will be something like VM123456-2G and/or VM123456-5G. Connect to the -5G network for best results.
To reboot, Virgin recommends that you turn everything off and wait for 60 seconds. Then turn on the Super Hub; wait another 60 seconds, then turn on the computer.
21.1 Mbps seems a bit slow for Virgin SuperFast Broadband - their lowest fibre optic product is 'up to' 50 Mbps - I rarely see less than 53 Mbps via WiFi.