Install packages:
opkg install comgt kmod-usb2 kmod-usb-ohci kmod-usb-uhci kmod-usb-serial kmod-usb-serial-option kmod-usb-serial-wwan kmod-usb-acm usb-modeswitch usb-modeswitch-data luci-proto-3g
Edit /etc/config/network and add 3g UWAN (UMTS WAN) interface:
config interface 'uwan' option ifname 'ppp0' option pincode 'XXXX' option service 'umts' option proto '3g' option device '/dev/ttyACM0' option apn 'YOUR.APN' option pppd_options 'debug user "" password "" noipdefault refuse-chap refuse-mschap refuse-mschap-v2 refuse-eap'
Then start interface with:
ifup uwan