this will not work.

when you are online over gprs, you don't really have a public IP. you get a private IP inside your carrier's network, and every connection to the outside is "masked" with the carrier's public IP. hence many, many devices appear to have the same public IP.

the only way to make it work over gprs (other than signing an enterprise-level contract with your carrier to get a real public IP) is to set up some sort of virtual private network (VPN) to a trusted network. then you could access the device directly from that trusted network.