In order to access a cellular router (or a device connected to it) remotelly, using the router's mobile interface IP address, it is necessary that the IP the carrier has assigned to the device be either public, or private with a private APN.
If the IP assigned to the device is private and it does not have a private APN, then the cellular router will be unreachable via its cellular IP. In this case the the only way to reach the device remotely is if the device initiates a VPN session with a server.
If the mobile interface of the device has an IP in one of the following ranges (private IPs), then a private APN is required to reach the device remotely:
Private IP ranges:
192.168.0.0 - 192.168.255.255
172.16.0.0 - 172.31.255.255
10.0.0.0 - 10.255.255.255
The cellular carrier needs to know what kind of service you need in order to provision correctly the service and select the correct APN.
Jan 09, 2020