WhatsApp for Samsung Galaxy Core II

First rule out the cheap causes: gently clean the micro-USB port with a wooden toothpick or a quick blast of compressed air, since lint packed into the port is a very common reason charging stops. Then swap to a known-good cable and a basic 5V/1A wall charger (the Core II doesn't fast-charge, so avoid odd high-output bricks) and try a different outlet. If it still shows nothing, pull off the back cover, reseat the removable battery, and leave it on the charger for 15-20 minutes before expecting the screen to wake. If the battery is several years old it may simply be worn out, and a fresh replacement battery often fixes a Core II that no longer charges. See how others solved this on Reddit

The Core II ships with Android 4.4 KitKat, and current versions of Instagram, Facebook and WhatsApp require much newer Android, so the Play Store will block the latest builds. WhatsApp dropped support for KitKat devices, so the modern app won't activate. Your realistic options are lightweight 'Lite' clients (such as Facebook Lite) where still available, or accessing these services through the phone's browser instead of native apps. See how others solved this on Reddit

As a basic phone it still works fine for calls, SMS and light browsing, but the 768MB of RAM and dual-core CPU mean modern apps stutter or won't install at all. To keep it responsive, go to Settings > Application manager, swipe to the All tab, and Disable bloatware you don't use; you can also long-press the home screen to remove heavy widgets. Treat it as a calls, texts and light-browsing device rather than a smartphone for app-heavy tasks. See how others solved this on Reddit

The Core II only has about 4GB of internal storage, so it fills fast. Insert a microSD card, then go to Settings > Application manager, open each large app, and tap Move to SD card where that button is available; you can also set the Camera to save to the SD card via the camera's settings gear. Clear cached data under Settings > Storage > Cached data, and move photos and downloads to the card to free up the internal partition. See how others solved this on Reddit

TouchWiz on the Core II includes a simplified layout: go to Settings > Easy mode and switch it on to get large icons, big fonts and a stripped-down launcher with favourite contacts on the home screen for one-tap calling. You can also raise the system text size under Settings > Display > Font > Font size. This keeps the interface to the essentials without installing any third-party launcher. See how others solved this on Reddit

The Core II makes a decent 'dumbed-down' phone if you tame the smartphone features: turn off mobile data when you don't need it, disable unused apps under Settings > Application manager, and remove distracting widgets from the home screen. If you mainly want calls, texts and the occasional map or browser, the Core II covers that without buying new hardware. Only move to something like a Galaxy Folder 2 if you specifically want a physical keypad and flip form factor. See how others solved this on Reddit