Its all very easy, simply install this Alternate Apps store - Aptoide.apk using a File manager and USB Stick.

Then install Apps directly from Aptoide, you can even update them from Aptoide.

https://m.aptoide.com/installer-aptoide-tv?lang=en

BTW: Use X-Plore File manager (from ATV Playstore). ES File Explorer has been known to cause a bunch of problems on Android TV devices. Its now the worst Bloatware, Adware, Spyware crap you can install.

Download the apk to your phone, install es on both, transfer from phone to mibox via es. Then uninstall es, because it's easier to reinstall it than it is to turn off all the trash features with a remote control

Download Puffin TV from the Play Store and there you can use Google to search for Aptoide TV. Install it and download apps that are not in the TV Store from there.

Not without at least a mouse connected

you can try to install FireDL from playstore. you can enter the file url or enter the fireDL code. you can check the list of apps you can install using just the codes here: http://radianware.srve.io/apps.php

You'll still need a computer to find the URL, but an app called #SIDELOADTAG combined with a tinyurl short code to a direct apk will download the apk to your device and let you install (assuming you've set unknown sources to allowed of course). For example entering the code FFXBTA (that one's a vanity code, most others would be more random) will download one of the Beta channel releases for Firefox (which will require a mouse to properly navigate as it isn't dpad optimized)

if you can connect a mouse install Chrome. Then you can download and install apks right from the web

Aptoide.apk