Father-in-law's uses Mobdro on his TV, It stopped working and needs an update, However when trying to update the INSTALL button is greyed out.
Uninstalled the app and tried reinstalling and the install option appears to be grayed out for all APKS.
https://i.imgur.com/WeBQfft.jpg
EDIT: Spoke to Philips who informed me they have released an update that fixes the issue of installing from APK, I had received a software update that caused the bug but had not yet gotten the next update which solved the bug.
All is now working again as it once was.
I don't know if it's just the angle, but the screen looks weirdly blue. If you have any kind of screen overlay installed/enabled try disabling that as anything that covers system prompts will prevent them from working as being drawn over they could be used to trick someone into installing something.