Hello Rexter.
Did you see above where I stated I had enabled developer mode?
Also, may be nothing, but I noted you called it a phone. It is a tablet, for sure, but android thinks it's a phone...
It doesn't matter if it's a phone or a tablet. The setting to install unknown apps can be located or named differently, depending on the device and the version of Android that it's running.
Modern versions of Android let you search for...