When I add dun through the app it doesn't reflect in my apn settings, even after I reboot. I systemized it into system/apps.
if i was you try offline sim apn database make it a system app ive done it and then you can add dun apn but change to nxtgenphone apn use edxposed get download xtether and moto x hide that used to work i was red flagged on cricket with my s5 back in the day