STEP 1:

Insert the 3G modem on your PC and disable the SIM PIN code. This is usually found under Tools>>Options.

STEP 2:

Insert the 3G modem on your router.

STEP 3:

In web browser (IE or Firefox) access the menu page of Vigor2820 series (default IP address is http://192.168.1.1) 

STEP 4:

Go to WAN > General Setup page

In WAN2 

  1. Select Enable: YES  
  2. Select Physical Mode: 3G USB Modem

STEP 5:

Go to Internet Access (WAN >> Internet Access) page

Select WAN2

STEP 6:

Enter the following details

  1. PPP Client Mode: Enable
  2. APN Name: telstra.bigpond
  3. Click “Apply” button
  4. Enter your PPP username and password

Click “Ok” button

Before:

After:

The router will reboot after Clicking “Ok” button.

Wait for 10 seconds then click on the link to connect to the router again.

STEP 7:

Go to Internet Access (WAN >> Internet Access) page

Check that the details shown are similar to that below.

STEP 8:

Power off your router, connect the 3.5G HSDPA USB modem to the USB port and power on.

You can also check your online status in Vigor2820 or check ACT status on the modem.

Note:  A video of the configuration steps is available at: https://youtu.be/b4pzFXUvfOA