Does your mobile interface have an IP address? You can check it via WebUI by navigating to the Network>Interfaces and checking whether mobile interface has an IPv4 address assigned. If not, I'm leaving a potential solution below:
In the WebUI, navigate to System>CLI and login to the router via CLI (SSH) by entering the following details:
Password: <WebUI router password>
Details about logging into device via CLI can also be found on our wiki page here: https://wiki.teltonika-networks.com/view/RUTX14_CLI
Once logged in to the CLI, execute the following commands in order
gsmctl -A at+cgdcont=1
gsmctl -A at+cgdcont=1,\"ip\"
ubus call gsmd reinit_modems
Once done, the router should obtain its IP address almost immediately. Let me know if this solves your issue.