Catching up with the forum now, apologies for not getting back to you, was off sick recently.
TL;DR? App installed and updated (HAVE to use the DroidBOX Market option when offered). Don't have an account to test with, tried the free 7 day option, didn't work. Set VPN to USA and the application proceeded. No video playback testing completed, so can't guarantee it'll work.
T8-S Plus, installed the APK linked to above, no errors reported during standard installation screens.
Opened up for the first time, it told me I needed to upgrade to the latest version before proceeding.
OK button was only option, it then asked if I wanted DroidBOX Market or Google Play Store to be used.
Play Store came back with "This item is not available in your country"
Tried DroidBOX Market, which gave me Open/Update/Install options. Uninstall not going to help, open would just loop round again, so went for Update. After downloading it went through the standard Android application install/update screens, updated OK.
As I don't have a Sling account I couldn't sign in, so went for the Watch 7 days FREE! option.
more to come, posting now so I don't lose screenshot or text.
Tried the same with a VPN in America set up, it progressed:
However, I have not signed up for an account (need to enter billing details), so can't guarantee that actual video playback will work.
If you've another Android TV application that can be tested, that doesn't require credit card details to be entered, let me know and I'll test again.
[edit]
If you've a login for the service, and can change the password afterwards (for your own reassurance), I'm happy to enter these, shoot me a PM if you want to try this. For testing purposes only, will uninstall afterwards!