Did you set the "Check interval" on both SIM cards as well as selected an action on both SIM cards? It needs to be set on the secondary SIM as well to make sure the router knows what to do in case the primary connection is back online.
If that doesn't help, could you please generate and send me over a troubleshoot file via private message?
What's a troubleshoot file and how to generate it?
A Troubleshoot file contains a device's event logs, configuration files and other info useful for diagnostics. It can be downloaded from your device's WebUI, Troubleshoot page:
System → Administration → Troubleshoot
The load balancing can still be achieved by having one active WAN (wired) interface, one active WWAN interface (WiFi WAN) and/or one active mobile interface. The only device that is able to accomplish load balancing with both SIM cards simultaneously is the RUTX12. If you have more questions regarding this topic, though, please create a new thread as this is a slightly different issue.