Check these in order:
Mobile Data is ON: Settings > Mobile Data > Zable > Mobile Data ON (iPhone) or Settings > Network & Internet > Zable > Mobile Data ON (Android)
Data Roaming is ON — yes, even in the UK. Settings > Mobile Data > Zable > Data Roaming ON. Despite the name, you won't be charged roaming fees — this setting is required for your eSIM to work.
Check your data usage — open Zable app > Usage. If you've used your allowance, you'll need to add a top-up.
Turn off VPN — VPNs can block mobile data. Temporarily disable any VPN apps and test
Check APN settings:
iPhone: Settings > Mobile Data > Mobile Data Network. APN should be gigs, username and password blank.
Android: Settings > Network & Internet > Mobile Network > Advanced > Access Point Names. Select "Gigs" or create new with APN gigs, all other fields default.
Still not working? Work through the "No Service" steps above, then email zable-SIMcard-support@zable.co.uk. from your account email describing your issues, who will respond within 4 business hours.