How to set an app to run 24/7 in the background?

I think I've tried all the tricks (allow background activities, power saving mode off, sleep standby optimization off, app lock) but I still can't get my app to run in the background without getting "killed" by the OS. I've done lots of reading and there is another trick about Auto Launch, or Auto Start but my phone global ROM Find X8 doesn't seem to have that option anywhere in the settings. Anything else I can't try?

Thanks all.