Hello,
According to information which you have attached as screenshots, there should be no issue for your DL8 device to reach internet trough RUT240.
To avoid any plausible misunderstanding due to interfering past configuration or firmware version, we would suggest to upgrade your RUT240 device to latest firmware version without keeping any previous settings. Latest RUT240 firmware can be downloaded from here:
https://wiki.teltonika-networks.com/view/RUT240_Firmware_Downloads
Then try to check if your DL8 device is working with RUT240, while as much as possible RUT240 configuration would remain in default values (except mobile configuration, since without APN device might not connect to the network). If possible, also try to temporarily switch your DL8 device to 192.168.1.0 network for testing purposes.
If solution would be working with RUT240 in default configuration, the continue to configure router to its previous state (previous configuration), periodically checking if your DL8 device is still receiving internet connection. If you would encounter any issue during configuration, that would allow you to know which exact configuration aspect causes DL8 to loose internet connection through RUT240.