No idea. At this point you may want to go back to DrayTek and ask them if there is a setting either in the router or extender that needs to be changed. Or spend some time digging through the user manuals for both units to see what options and settings each has and see if anything could be blocking communication between the two devices. Maybe you need to check the UPnP, SSDP and if any local network media sharing ports are being blocked by either device.
As previously suggested there are other options you could look at. For example reconfiguring your network by moving devices around so the Pioneer and My Cloud are both connected to either the router or to the extender is one such option.
