IOS – Set iOS device into Kiosk mode and get the screen not to “sleep”

I've got an iPad Mini 4 running iOS 12 in a in-wall dock that I want to leave locked to one app and always on. The wall dock provides power to the iPad.

To have the iPad locked to one app I put it into Kiosk Mode.

I have set the settings for auto-lock as per the screenshot below, but I can't seem to get it to stay constantly on i.e. not to go into sleep mode.

I've had this working fine on other iPads using the same spec wall docks in the past, but I can't get it to work the latest time I've tried to deploy this. I think its got something to do with iOS 12 as my previous installs have been running iOS 10 and 11, and because they are locked into Kiosk app mode the OS never gets updated. I have also tested on my personal iPad mini 4 (iOS12) by plugging it into the power, setting sleep settings as per the screenshot above, and putting the iPad in kiosk mode locked to one app, but after about 30 mins it locks / dims the screen, it does seem to "wake" again if I get an alert on that device, but it only lasts for another 30 or so minutes.

Any ideas how I can get this to work so that the iPad will be locked to one app and the screen will stay on constantly?

Ideally I don't want to do this, but if necessary I would be willing to lock the device down in some sort of enterprise management mode where I can have more control over the device instead of using the Kiosk mode "hack" approach above if it resolved the issue.

(People may say that iPads are not designed to be always on/always charged, but I've had a few installs like this running for 2+ years with no issues).