Thanks @wellnw.

Please log as feature request for this:
If cable connection is not available (cable still can be connected to upstream, but external hosts may not be reacheable),
I want router to establish LTE connection and failover to it with periodical checks for cable connection to become available.
Following settings should be available: upstream connection check interval, number of failed check before failover, main connection check interval when failed over, number of successful checks before switching back to main connection.

Thanks