Hello,
Load Balancing can only work between two active interfaces. As RUTX50 has a single modem, it will unfortunately not be possible to achieve it between the SIMs. You could, however, use a wired WAN connection along the SIM inside the RUTX50.
The only device that can load balance connections between 2 SIMs is the RUTX12, which has two integrated modems, thus both of the interfaces can be active at the same time. More information about RUTX12 can be found here.
Let me know if you have any further questions!
Best regards,
DaumantasG