Once the subscription is added to Happ: open Happ, pick a server from the list on the main screen and tap the big connect button in the centre. Done — your traffic goes through the VPN 🌊
No connection? Refresh the subscription in Happ — the ⟳ icon in the subscription header — or pick another server. You can also re-add the subscription from the web account (lk.surfingvpn.com): the "Connection" tab → "Connect".