HiMedia S500 comes with Android TV Play Store pre-installed. This works well but Android TV Play Store only features apps specifically designed and certified for Android TV, so many standard Android apps are missing.
However, S500 can run mostly all Android apps. This thread details how to install non-Android TV apps on an S500: https://www.futeko.com/newforum/index.php?topic=4697.0
If you want a simpler way to do it you can install an alternative app store. These work a bit like Play Store but are much simpler, being just an index of Android apks rather than requiring the installation of Google Play Services ecosystem.
https://en.aptoide.com/ (General app store with all big apps - the closest replacement to Play Store)
https://f-droid.org/ (Free open source Android apps)
https://www.getjar.com/ (Another general app store)
Note that these apps require a pointing device (USB mouse/airmouse).
1. Install a file explorer. Try X-plore from Android TV Play Store. It works well with S500 remote and is very comprehensive including SMB access.
2. Change settings to enable what Google calls 'unknown apps'. Go Settings - Device Preferences - Security & restrictions - Unknown Sources. Find X-plore (or whatever file explorer app you installed) and set the toggle to on.
3. Download Aptoide (or alternative as above). https://en.aptoide.com/download
4. Open X-plore (or other file explorer), find the Aptoide apk file you downloaded (insert USB stick first if you have the .apk on a USB stick!) and press OK on remote.
5. The apk will install just like any other app.
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