Ever since upgrading my HTC Desire HD to the latest version of Android (2.3.5) I have had no 3G mobile network. This just requires a simple fix to set your mobile settings to its default. Simply follow the next few steps to fix the problem.
Reset HTC HD Desire Android Mobile Settings
Click on Menu Button on your phone
Click on Settings
Click on Wireless & Network
Click on Mobile Networks
Click on Access Point Names
Click on the menu button again which will bring up options on the bottom of your screen.
Click Reset to Default
That’s it you should now have your mobile network working again.