Wireless modem for S7

I have and S7 CPU with ethernetconnection and an MP377 on ethernet. The customer is not willing to allow me VPN-entrance to the controller. Is there a way to use a modem f.i. gprs that can be connected to the ethernet of the S7, while I on the other side can connect by the internet?
Thanks

I have used gsm modem from this company with good results. Siemens has products as well but they are more $$$.

Pro4 has a gprs/ethernet modem but i have never used this myself.

Here is an option more:
http://www.westermo.com/Resource.phx/content/products/ethernet/adsl-router/mr-200.htx