Certain apps do not connect to internet

Hey there.
My issue started a few days ago, when two apps(yes only two), Viber and Duolingo started acting up, and saying I don't have an internet connection even though I'm connected to my home WiFi. Every other app, browsing works perfectly. They worked on Mobile Data when I tried. And here's the catch. They DO work sometimes. In one out of 20-30 app starts they do connect and useable. But they start acting again.
Here's what I tried so far: Restarted the phone, removed the battery for a while, reinstalled the apps, restarted the router, checked my phone and router settings, reset my router. My WiFi is only used by me and occasionally one other, so it's not because it's crowded.
My phone is an LG Leon 4G (H340N).

Thanks in advance !