Internet access isn't the problem here mobile networks do not have the capability to receive incoming connections because they use a NAT network.
The only other possible way around this is to use a 3rd party ftp or web server i.e. homepage space given by your broadband provider.
You create a file containing the commands, upload this to your userspace and get the Pi to periodically check for the presence of this file.
When found the Pi would read and execute all commands then remove the file automatically from the server.
This method should work but given the fact you have to periodically poll the internet you're effectively eating away at your data allowance.
SMS or text messaging is probably a better and simpler way to control your Pi there is no need for internet access you send a text containing the commands from your phone to the number allocated to the data dongle (and this is how it should work).
A USB data dongle is nothing more than a dial-up modem with a virtual serial port (some have ethernet capability but not as common) you need to write a python script to communicate with it.
A dial-up modem is controlled by "AT" commands the examples I gave you in my previous post would display all received text messages.
All your script has to do is check for new messages, read new message, translate message into commands, remove text message, execute command as required.
If you still want to pass commands to your webserver then you can do this by calling the program wget i.e. wget http://127.0.0.1/cgi-bin/cmd.cgi
?(command string) etc.