Ubuntu MTS LTE - what's going on with the connection?

Question

Huawei

I don't understand anything...
Launched a 4G modem from MTS (Huawei E3372) under Ubuntu 14.04:
- switched the modem mode via AT commands to Modem/Network card
- registered in /etc/network/interfaces
iface wwan0 inet dhcp
- made a script to connect and registered it in rc.local

#!/bin/sh
echo -en 'AT^NDISDUP=1,1,"internet.mts.ru"\r\n' > /dev/ttyUSB2
ifup wwan0

After that, a couple of days everything started fine. The modem was connected via NDIS, the wwan0 interface appeared and the DHCP client clung to it, the Internet appeared. In general, everything worked as expected.
Strange things happened today after switching on. Instead of the wwan0 interface, ppp0 suddenly appeared! The NetworkManager has an icon for the MTS mobile network, and the connection information indicates that everything works through the GSM modem /dev/ttyUSB1.
My question is: how could this happen? Why does Ubuntu-ha go arbitrarily and do things that I didn't tell her to do? Where to dig and what to twist, tell me!