Android uses a managed store in which you can 'buy/select' the apps to install.
Some apps are not available in the office store. You will have to download a APK installation file from the internet and then install it manually. This is called side-loading. For Android TV this might also require...
Smart iptv also seems to be a good one but must be sideloaded as far as I know. Still on my list to try. You can try Perfect Player for free from the normal play store. Works great with the normal remotes.
You can attach an usb3 network connector to the MiBox. This also improves the network stability a lot. (Current official Android OREO firmware has wifi stability issues). I just bought a second MiBox for my home. IPTv works great with the app Perfect Player.