Home Network Switch Ports Issue

I have a 8 port switch that is connected to my DSL modem. There are 4 devices connected to the switch and all works well. I have a 5th device that is not cooperating- I use it for Plex media files storage (Its a DNS-323 dual bay, RAID enclosure).

Whenever I add the media file device, 2 of the 4 existing Ethernet devices disappear and are non-operational. The remaining 2 devices as well as the media device works perfectly- all files are available. When I remove the media device, all original four devices come back and work fine.

What’s going on? How can I troubleshoot? Thx.