I'm no longer using an 3G/4G card to connect, but my mom is and daily I have to go through this routine to re-connect:

Open network manager, click 'manage connections', delete the thing that was working, hit apply

next...go over and turn off the air card (or whatever it is you're using), turn it back on (sometimes I may need to do this a few times - not sure why)

next...back at network manager click on the drop-down menu and 'add' and 'wireless'.

click on the 'scan' and then highlight whatever it is it's found.

usually just clicking on the couple things to fill in works. Then go to the next tab for the password, enter it, click apply and then okay and hopefully that will work.