Apps abilities to run in the background depends on the app itself having such ability and more importantly, the firmware. Majority of these boxes almost always have a built-in task killer and some firmware/kernel can be quite aggressive in killing apps. Meaning you are out of luck and have to resort to other means. Try KeepRunning, and it doesn't seem to require root.

As for the iFlix issue, its an either it works on your box or it doesn't at this time. Up to iFlix to update it to work on your box. In the past, one was able to run older versions of the iflix app, but recently they've pushed out an update and using an older version to connect to their servers...results in the app requesting (forced) for the update. Tons of android devices out there, hard for their devs to keep track and can't really blame em as the app's originally designed and maintained for small screens.

this problem is also seen with 2 other local providers - astro-on-the-go and hypptv everywhere.

they work on some boxes, don't work on others.

but they all work on tablets and phones.

that says the providers either intentionally block usage with boxes or simply too slow to catch up with all kinds of android boxes in the use.

Star ok but WDTV3 not working on my 6.0. 

can i use unifi box instead? or the android box better?

Yes, that's how I started.
Bought a dedicated box now.

Honestly speaking, it's a not too bad box for basic usage. Even now I have a 32GB storage box, I don't stream those Kodi's movies add on all the time. Sometimes I'm wondering what to do with such big storage.
For future proof I guess.