1. First of all, please check if your RUT240 product code is exactly of this type: RUT240 *1**** (it's an AT&T, Bell and T-Mobile version of RUT240).
If yes, lets move on to the next step:
2. Ask your operator what APN are you supposed to use with your data plan, once you get it - go to Network > Mobile in WebUI and find Auto APN option. You have to untick it and type in the APN you were given into Custom APN option and save the changes you've made.
After some time, if that was enough, your mobile data connection should go up, if not - check the Network > WAN page if the Mobile WAN is selected as Main WAN.
If this also didn't help - lets move on.
3. Try reflashing your router without keeping the settings with the newest possible firmware in our wiki page: https://wiki.teltonika-networks.com/view/RUT240_Firmware_Downloads
Once you've done it - repeat step 2.
4. If the above also didn't help try using different SIM cards of the same operator or if it's possible try Bell or T-Mobile.
If nothing helped try downloading the troubleshoot file that you can find in System > Administration > Troubleshoot. Make sure you download it after about 5 minutes of routers uptime to make sure the device tried to connect to the network already.
You can attach the file here in this thread and it will be visible only to the moderators of this forum.