It is not clear how you have configured your network. Typically one connects a My Cloud to a “router” or “switch” not to a “modem”. Only time one would connect their My Cloud to a “modem” is when that “modem” is actually a router/modem combination device.
What is the purpose of using a second router? Generally adding a second router with the intention of creating a second IP subnet on a local network will require configuring the second router to properly pass data through the second router’s firewall. Not all router’s can be properly configured to pass data through its firewall.
If you do not need to have a second separate IP address subnet on the local network then simply access the second router’s administration screen and disable the second router’s DHCP server. Then connect the first router to the second router’s networking port not the second router’s WAN or Internet port. this basically turns the second router into another access point or basic network switch.
