Android phones, apps and tablets

As of Android 4.3, tethered traffic is tagged by default:

Untagging tethering traffic, 4.3+, 5.*: (Requires the device to be writable when connected via USB debugging)

adb shell settings put global tether_dun_required 0

Untagging tethering traffic, 6.0 onwards: edit the build.prop file to add

net.tethering.noprovisioning=true
https://android.gadgethacks.com/how-to/enable-free-wifi-tethering-android-marshmallow-0167832/

I've also read that adding "dun" to your APN type might help.