Because you are using two completely separate networks the My Cloud will be seen as a remote drive on the second network. As such one could use the My Cloud Desktop program, the WD mobile apps for iOS/Android, FTP, or MyCloud.com web portal on the devices on the second network to access the My Cloud (in a remote manor). Another option is to setup a VPN connection between the two separate networks so traffic an be passed between them. Or, and its been many years since I’ve had to do it so I don’t quite remember exactly how it would possibly be done, you might be able to connect the two networks together (via Ethernet) and configure the router/gateway of each network to pass the traffic from the other network.
You may find additional information by performing an internet search since this is specifically a networking configuration problem and not a My Cloud specific problem.