Hello!
As I can see from the topology, the solution assumes the operation of two WAN interfaces at once, which is unfortunately impossible on Teltonika devices. The Load Balancing functionality works only for switching interfaces according to the specified rules.
However, I think you can try to work around this. And solve the issue with additional settings.
If you do not use a wired WAN for any other purpose, then you can assign the use of this port as a LAN in the settings, as described in the instructions.
Then connect the second router to this port, thereby adding it to the RUT240 LAN network.
And the next step is to set up traffic rules in the firewall to redirect certain traffic to the second router. Learn more about the firewall and traffic rules before configuring it.
Best regards, Aliaksandr!