This is a topic of the EtherCat functionality and the S20 bus couplers. You cannot scan "throug" devices, when they are not known in the bus. This will be different in the ctrlX IOs because there each IO is an own slave.
You have to scan once the coupler then download the EtherCat configuration to the ctrlX CORE. Afterwards simply scan again and the sub modules will be found (if you have problem at the second scan try to switch EtherCat state to Safe-Op or Pre-Op).