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Kindly check the following.
1. Have you selected the LLS mode in the RS485/232 settings?
2. Please check the datasheet of the LLS sensor and look for the baudrate setting.
3. Check the sensor datasheet if your connection is correct.
RX to TX
TX to RX
A to A
B to B
4. For your reference: FMC640 RS-232 and RS-485 - Wiki Knowledge Base | Teltonika GPS (teltonika-gps.com)