No connection to WD EX2 Ultra

Hi all,

Form my Macbook I cannot connect to MyCloudEX2Ultra when being in the room where the main router is positioned (“The server may not exist or it is unavailable at this time. Check the server name or IP address, check your network connection and then try again”). However, when being upstairs and downstairs, where Fritz extenders/routers are being used, in different rooms I can access MyCloudEX2Ultra. The same applies when trying to access via the IP addresses of the devices - as well as accessing the Fritz Extenders.
I have tried to turn everything off (router and devices) - no luck. Besides doing this all the time the Sonos system then also interferes.
Thus is there anyone out there to help, i.e. how can it be that access to devices is possible through extenders (being in another room) but not when directly connected to the main router?