Hello,
You could try to check if your RUT DHCP server is enabled by navigating Network > LAN. Also, try to add a static lease for the Cisco device in the same configuration window. Additionally double-check if you connect devices correctly (Ethernet cable should go to RUT LAN port and Cisco WAN port).
Moreover, please use the latest firmware version: RUT9XX_R_00.06.08.2.
Regards.