The user that can become root (admin) is the user you created during installation. So as guest you wont be able to install new applications or modify anything in the system. You wont even be able to use the sudo command (Super User Do).
To troubleshoot your USB internet you will need to be under your user account and issue command using the sudo.
A quick fix would be if you log in to a terminal so at loggin press Ctrl+Alt+F2
Then type your user name hit enter then type your password, You wont see your password characters being typed so dont worry about it, hit enter once you are done
Then create the new user with the command (Press enter after typing each command)
sudo useradd john123 -m -s /bin/bash
Then set up a password for that user with the command
sudo passwd john123
Then add that user to some groups with the command
sudo usermod -aG adm,cdrom,sudo,dip,plugdev,lpadmin,sambashare john123
Those groups are the one your user will be able to use under that user name
reboot the computer
after that and you will have that new user name at the loggin screen.