The BR1 will failover to the other SIM when there is no working internet connection.
This could be because of a failed heath check or because of a SIM that is corrupt or out of data.
You could also configure a cellular signal threshold to force a failover when a cellular signal is below a certain value as explained in the video below.
Someone else raised a similar question in this post. @SteveTaylor replied to that question with a few more useful details.
