First is that NetworkManager does not save chap/pap settings. This is a NetworkManager bug. But it can be fixed in /etc/ppp/options:

sudo sed -i 's/#-chap/-chap/' /etc/ppp/options

This will disable chap authentication globally for all connections that do not override the option (such as NetworkManager).

Second thing is that the APN is wrong. I don’t know if this is only for pre-paid, as that is all I have tried.

After plugging in the USB adaptor and selecting your internet provider:

• Right click the NetworkManager icon in the system try
• Select the Mobile Broadband tab
• Click on your connection then click Edit
• Change the APN from VirginInternet to VirginBroadband
• Click Ok, then Close

To start and stop data sessions now, left click the NetworkManager icon.

If your are asked for a password to connect, anything can be entered.

(ref)