Based on the troubleshoot file, the gateway itself is able to establish and sustain mobile connectivity.
However, the troubleshoot file indicates rather low mobile signal characteristics:
RSRP: -112 dBm, RSRQ -16 dB, RSSI - 75 dBm, SINR - 0 dB.
I would suggest to try changing device's location, if possible, attach a different antenna.
You can also try setting different frequency bands manually. Instructions, how to do it are provided here. Bands to try are: B1, B3, B7 (current), B8, B28.
You can track signal indicators in the WebUI Status -> Network -> Mobile section. For reference values, check this page.
Network -> Interfaces -> LAN interface IPv4 gateway value should be left as empty.
Another suggestion, that comes from the file is to update your device's firmware, which is currently TRB1_R_00.07.01.2. You can download the latest version from here and update via WebUI, by navigating to System -> Firmware. Before uploading the file, disable Keep settings option.