wireless PLC access

I try to connect to a PLC via 4G usb wireless adapter. My setup is a PLC, HMI,
Craddlepoint routerMBR95 and a T-Mobile Rocket USB adapter.
I cannot ping the IP address on the USB stick or the modem. I can access the
internet from my laptop via the router/T-Mobile wireless adapter. When using
whatsmyip.org it seems to give me the IP of the cell signal company at their
location. When I ping that address from another PC I get a reply but when turning off the router it still works telling me that it is not on the router.
The IP that the router displays does not ping.
What am I missing?

I am only guessing, but the USB 4G wifi unit is for access to the internet.
You may need to talk to your WIFI provider.

My goal is to connect to the PLC/HMI remotely via a cell phone network,
like T-Mobile 4G data service. I'm trying to test this while having the system
at the same location. My suspicion is that the T-Mobile USB device switches to
using my Wifi network when I connect it to the router. As I need to keep my Wifi
up trying to connect this might be the culprit. The USB device always defaults
to Wifi not broadband and only on my laptop can I change the selection not
on the router.
I'm looking for a simple solution to use cell service to connect to HMI/PLC where
regular Internet access is difficult to get.

Are you trying to get remote access from the outside world into the PLC via the 4G network? Unless you can get a 'static' IP from T-Mobile, you'll never know what the IP really is... There are Dynamic DNS services that might work, but then you'll need an intermediate PC or appliance running locally to manage that service.

In any case, it's been my experience that mobile carriers never give you a "public" IP address anyway... that way it makes the Internet connection a lot more secure from hackers and such. (You'll likely get an address in the 10.xxx.xxx.xxx range, which is NOT available to the outside world)

I might be off a bit in my description, but I think what you are trying to do is quite a challenge.

Basically we plan to have data connection to a PLC/HMI via the broadband cellular network. I've tested wireless modems at 3G speed (Sierra Wireless Airlink) and the transmission was slow. They also cost in the range of $500 plus the data plan. I'm looking for a more economical solution with higher data speed
for programming and HMI control.
My next option is using a simple PC with a very small footprint. The goal of all this is being independent of the local IT network or have access when an internet connection is not available otherwise.
I see a market for this and hope to find a simple, reliable solution.

There are various options to do what you want, the only bit they fall down is the 'cheap' that you are looking for. My preferred choice (take of the *s in the following web address)
www.e*w*o*n.biz

These gives remote access, if you choose the one with a SIM slot it can use 3G. I can't remember if it provides failback from the Ethernet WAN port to the 3G system. It is a small form factor computer and I think that it is running Linux as its operating system.

If you search for 'remote access' on this PLCS.net website you will find lots of threads.

In Australia you require an M2M SIM card (machine to machine) - the modems for this type of communication is only available in 3G I believe.

Most air cards use NAT so that the IP you get is not a public IP but a dynamic (changing)block.

Most carriers will give you a static IP for 3-5 dollars extra per month. Contact you carrier about a static IP.

Some air card routers have built in provisions to use dynamic IP services like No-IP and DynDNS,etc.

A computer connected to the cradlepoint could employ a dynamic DNS service also.