After searching, it turns out that the 4G operator gives a private ip address to the sim card and then there is a routing that is done by the operator.
Example: IP Phone Address: 10.90.xxx.xxx IP Address my-ip.com: 80.12.xxx.xxx same as the modem.
The Teltonika RMS service accesses my router from outside, so why not access it with the openVPN server ?
thanks in advance for your answer