2 Router Configuration with WD Cloud Help

I have a sonicwall TZ100 connected to cable modem in my upstairs office. It is connected via hardwire to 2 PC's, Printer and Cisco/Linksys EA4500 wireless AP which is located downstairs. All of the ports are being used on the SonicWall. The sonicwall delivers DHCP to upstairs devices on a 192.168.2.1-100 network. The Cisco/Linksys located downstairs has hardwired PC connected to it as well as wireless devices it deliver DHCP on a 192.168.1.1-100 network. I recently purchased a WD Cloud Backup device and connected it to the Cisco/Linksys device. I can access it from the internet and can backup the PC connected to the Cisco/Linksys AP but can not connect to the PC's connected to the Sonicwall.

From my reading it seems that the Cisco/Linksys AP should be set to be a switch but if I recall I need it to act as a router to deliver DHCP to the various wireless devices, unless I set everything to a static IP which is a pain for wireless devices.

I am out of ports on the Sonicwall. Should I buy a small switch and plug that in upstairs off the Sonicwall and then plug the WD Cloud Device into that switch. If I do will I be able to access my downstairs computer on the 192.168.1.1 network.

Thanks for your help and advice.

Pete