First Download this Openwrt image ( compile by khairilyazit  & Ibnuasad  )
-> openwrt-ar71xx-generic-tl-mr3420-V1-squashfs-factory.bin!

now follow all these step below with LAN CABLE CONNECTION ONLY:

- go to your browser login 192.168.1.1
- sign in
- go to system
- firmware upgrade
- click browse ( find the openwrt firmware u download just now )
- click upload
- wait for uploading firmware finish and it will auto reboot ( make sure dont power off the router when this period because it will brick the router )
- once update % finish, try to close your browser and clear your browser cache
- then login 192.168.1.1 ( you will see LuCI Gui interface )
- click login
- go to System
- go to Administration
- key in your new password ( For web Gui )
- re key in your same password again
- click save and apply
- go to Network
- click Edit on Wan ( 3G wan )
- select UMTS/GPRS/ED-VO protocol
- click switch protocol
- select UMTS/GPRS/ED-VO on protocol again
- Modem device ( zte mf680 = /dev/ttyUSB1 , E372 = /dev/ttyUSB0 <- last is zero )
- service type : select UMTS only or UMTS/GPRS
- Key in your ISP APN , ( Unet, Maxisbb, bbnet, celcom3g, u3g, my3g and so on )
- Go to ADvance setting
- Modem init time out: 5 ( key in )
- Use gateway Metric : 0
- Tick the box of : if uncheck, No default route is configure
- Use custom DNS server : 8.8.8.8 click the icon at the end
- second roll: 8.8.4.4
- LCP echo failed Threshold : 0
- LCP echo interval : 5
- Inactivity timeout: 0
- select Wifi
- click Edit
- select your wifi prefer channel 1~11
- enter your wifi name in ESSID
- go to Wireless security
- encryption : WPA2-PSK
- key : wifi password
- click save and apply
- then click enable to on your wifi ( if u need it )
- now go to System
- Reboot
- click Performs reboot
- plug in your Usb modem after
- wait for reboot finish and enjoy

- login 192.168.1.1
- your password
- System
- Schedule Tasks
- type in or copy & paste this command : * * * * * /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
- Click Submit
- Go To Reboot
- Select Performs Reboot

After  that u can check the ping whether working, -> Status -> System log:
u will see something like this meaning it is working : 

Oct 30 05:43:10 OpenWrt cron.info crond[1140]: crond: USER root pid 1732 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
Oct 30 05:44:01 OpenWrt cron.info crond[1140]: crond: USER root pid 1857 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
Oct 30 05:45:01 OpenWrt cron.info crond[1140]: crond: USER root pid 2441 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
Oct 30 05:46:01 OpenWrt cron.info crond[1140]: crond: USER root pid 3115 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
Oct 30 05:47:01 OpenWrt cron.info crond[1140]: crond: USER root pid 3408 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null
Oct 30 05:48:01 OpenWrt cron.info crond[1140]: crond: USER root pid 3414 cmd /bin/ping -q -s 1 -c 60 8.8.8.8 & > /dev/null