Just an update...
Sideloaded Ultraflix now perfectly works on my X850C. (probably because of recent update)
Here are the steps to follow using your computer:
1. Go to https://www.ultraflix.com/
2. Do either of these to download the app:
a). Search for "Android App Direct Download" on this page and click on it OR
b). Paste this link in your browser. http://www.droidbin.com/p19m6er5hcrb59eu5rtk11ul33
3. Format a USB in FAT32 format.
4. Transfer the app to USB. Plug USB to TV
5. Go to Home Screens -> Settings(at the bottom) -> Personal (at the bottom) -> Security and Restrictions -> Unknown Sources -> Turn this ON
6. Open the contents of USB using File Explorer app. Click on the UltraFlix App and Install.
7. Use Sideload Launcher app to open UltraFlix.
8. For First time login, use the TV SideView App on your phone and enter login details (will need to use pointer to enter each and every character...the virtual remote on the app won't work)
Others: Please add if I missed something
Edit: Clicking with physical remote won't work for the initial screen. Use the TV SideView app on your phone - download if you don't already have. Go to the remote option on the app, click on the mouse pointer - (looks exactly like your computer mouse pointer) Use that mouse pointer to click on the keyboard that's displayed on screen. Once you go past the login screen, you can use regular remote.