i used this app to make my Huawei 3g dongles work on my nexus 7 and it is very simple just install the app and enter your apn of your operator connect your dongles via otg cable and it will work the app available on google play store called ppp widget here is the link it is for rooted device
https://play.google.com/store/apps/details?id=de.draisberghof.pppwidget
https://play.google.com/store/apps/details?id=de.draisberghof.pppwidget