I would advise you to check if the device has internet connectivity and this can be achieved by doing a ping test in the CLI of the device. Ping 8.8.8.8 and check if you are getting a reply. Here is a link for more information on how to use the CLI Command Line Interfaces - Teltonika Networks Wiki
The second thing is to check whether the correct DNS settings of the active WAN are entered. In the case of Mobile WAN as the active interface, this can be done in the web UI Network>Interfaces select mob1s1a1 pencil symbol and navigate to advanced settings as shown in the diagram. In the example, the DNS area is empty because the ISP provides the DNS server IP