Thank you for the provided information. There could many issues why you might be encountering these kinds of speeds. Strongly recommend following this topic to learn more: https://wiki.teltonika-networks.com/view/LTE_Speed_Discrepancies
We recommend testing the speeds with a similar device (maybe try testing another LTE CAT 6 device) in the same location and with the same settings to see if there will be any improvement.
Furthermore, potentially Load-balancing feature was configured improperly thus you haven't encountered full capacity of the RUTX12, and please keep in mind that load-balancing do not increase the speed of LTE, but increase the throughput.
If looking for device with higher LTE CAT category, you could try and use RUTX14 which has LTE CAT 12 module.
If you have any issues and want to return the device, please contact your reseller.