Remote Access - Cellular / Dynamic DNS

Looking for alternatives to remotely access PLC's out in the field. Normal method is a Cellular Router that is provisioned with a Publicly Accessible Static IP Address. In looking at alternatives I was thinking a Dynamic DNS might be an option. In keeping with a Cellular Router I don't think this is possible however. If I use a standard provisioned Cellular SIM these are not publicly accessible correct? For example you cannot connect to a Cell Phone even knowing its IP Address if I'm not mistaken so this would be a similar setup if the SIM is provisioned without the Publicly Accessible Static IP Address.

The alternative is to have your cell router VPN into your office or a server elsewhere, otherwise you're right. I tried to do dynamic DNS with an AT&T SIM only to find that I was assigned an internal 10.x.x.x address. You have to pay extra for the static IP if you want to access it by an IP, AFAIK.

In that manner the PLC would initiate the remote connection correct?

We have used the RMV-531 for this, it can be configured to point to your server, or you can use no-ip service. You'll have to install a program/driver on your server (Windows) to make the connection. Your application will connect to the virtual comm port or IP provided and it can read the data from the PLC. The driver will be checking the connection and reconnect when the IP of the SIM changes. So you don't need public IP from the provider.
http://www.icpdas-usa.com/rmv_531.html

Cell router should have VPN client functionality that is set to autologin once it's connected to the cell network. If you have a cell router with digital I/O I think you can trigger the login that way on some cell routers, or tell the cell router when to login via serial connection even. Don't quote me on the last two though, only sure about the 1st. I've done the 1st on a Phoenix Contact modem. Red Lion's modem can do this too.