IOS – Force apps to not suspend, auto restart apps

I have at least two apps which must run all the time in background, otherwise there is no point about it:

1. HID Mobile Access: Opens doors via Bluetooth LE, when phone is locked/in pocket
2. Noke: Opens my bike lock via Bluetooth LE, even when phone is locked/in pocket

I try hard to not kill these apps; I always leave them running and they are in the apps list when I double tap the home button.

However, both apps I mainly need twice a day (morning/evening) and in about 50% of the cases it just does not work. After minutes of desperation, I take the phone out, unlock, restart the apps and it works. That's absolutely unuseable!

Background data refresh etc is on for these apps. I think iOS puts them to suspend so they do not respond via Bluetooth LE even though I haven't killed them and they are in the apps list.

I can see similar behavior with MiniKeePass which I like to keep open so I have quick access to my passwords. But sometimes I still need to re-enter my (long) password although I haven't killed it.

Also, there must be a way for apps to keep running: Whatsapp, Threema but also ParcelTrack respond even when I killed them.