Thank you for reaching us. To start off with your issue, I would first advice you to install our latest firmware version, which at the moment is RUT9XX_R_00.07.01.4. You can find the latest firmware in this website: https://wiki.teltonika-networks.com/view/RUT955_Firmware_Downloads.
Another thing, I would like you to do, in case the firmware update does not resolve the issue, is to check a few settings. Using the firmware, which does not work properly, the RUT9XX_R_00.07.01.2 - 4, connect to the router’s web interface. Check if the mode on top right corner of the WebUI is set to Advanced. If it says Basic, just click on the word to change the displayed interface mode. Next, on the left of the screen go to Network -> Interfaces and click on the pencil icon on the right of your mobile interface. A configuration window will pop up. Check if the option Auto APN is set to on and disable it. Several other options will appear. One of them allows to set your APN manually, pick your APN and press save & apply. Check if you have the internet access now.
Next, you can also compare if your router uses the same APN in both firmware versions and make changes on the latter firmware accordingly.
Lastly, if the issue still persists after applying the changes, please generate troubleshoot files from both firmware versions, the RUT9XX_R_00.06.08.6 where you have internet access and one from the latest revision, and send them to me. Troubleshoot files can be generated from the routers web interface by going to System -> Administration -> Troubleshoot and pressing Download in Troubleshoot section in both firmware versions.