Check these in order:
Settings > Mobile Data > Zable - make sure Mobile Data is ON
Enable Data Roaming (Settings > Mobile Data > Zable > Data Roaming ON). Despite the name, you won't be charged; this is required in the UK
Restart your phone completely (full power off, not just lock screen)
Toggle Airplane Mode on, wait 10 seconds, then off
Check Zable is selected as your default data line in Settings > Mobile Data
Still no connection? Email zable-SIMcard-support@zable.co.uk.