You haven’t told us a crucial detail: Your Fairphone OS version / Android version. The standard browser, Lightning and probably Opera Mini depend on Android’s built in browser engine.
When the browser (engine) gets outdated websites refuse to connect. Recently TLS 1.1 and below were deprecated and that’s why users must upgrade for secure connections.
If you are on an older version of Fairphone OS, this might apply for you too: ✏ Constructive tips for improved security for the FP1(U) Anyway, FP2 users are lucky because their phone’s software is still well-supported (yeah, I know, Android 7 is overdue) and can upgrade. You should always use the latest software version.
You can upgrade using the Fairphone Updater app.