In an attempt to clear device from tons of ads randomly opening and covering whole screen, I have uninstalled majority of preinstalled software (like TurboCleaner and MegaShop or something like that). After doing it there are no more ads covering whole screen, but also system UI is broken - there is no status bar (the top bar with notifications), software buttons are not present (device doesn't have hardware buttons), after installing app for software buttons I also discovered that showing all running apps (the square button) is not working (I think it was broken even before a restart, when status bar was still present). Tried installing Nova launcher in hope that it could fix the missing status bar, but even after toggling the option in settings it did nothing.
Any ideas? I would prefer not to do a factory reset - I think it will only bring back those full-screen ads displaying every few minutes, in lock screen and so on.
Device: Prestigio MultiPad Wize 3131 3G
OS: Android 6.0
The tablet is not rooted or modified in any similar way.