As the Smart Series dimmers do not use a signal cable for multiway switching, they have to be grouped using the Link Plus app. This involves creating 2 groups, one for switch and one for brightness. Additionally, the dimmers need to know which one is the master and which one is the slave. It is best to set the master to the one which is connected to the lights (load), and the slaves should be all other dimmers on the circuit. This allows the system to identify which one is which.
This video below shows how this can be achieved.