See your public IP address and which DNS resolvers are handling your queries. Verify that CleanBrowsing DNS is active and your browsing is protected.
The Play Store relies on to function. If you update the Store and it still doesn't work, you likely need to update your Play Services APK as well. Ensure both versions are compatible with Android 5.1 .
Before installing an APK, you must allow your device to install apps from sources other than the Play Store (ironic, but necessary). Google Play Store Apk Download For Android 5.1 1 BEST