Since you have replaced the controller, the new one is either in out-of-the-box condition and needs to have an IP address assigned, or it has an unknown IP address.
You can generally connect to it over Ethernet, or via the serial port, whose settings ought to be at the default 19200 baud, 8/N/1 framing, CRC error checking, and DF1 Full Duplex protocol.
You do need some basic Ethernet and TCP/IP networking experience to set up these controllers. No great IT skills, but the basics to understand cables and switches and IP addresses.
The "ETH" LED on the controller will be solid green if it has an IP address and is connected to a switch. If it is set for DHCP or BOOTP, it will make two quick blinks every 30 seconds or so when it transmits a DHCP/BOOTP request.
Because yours is a Series C FRN10+, it supports the modern "EtherNet/IP" driver in RSLinx Classic, which automatically browses the local IP subnet of the network adapter you configure it to use.
When I get one of these controllers out of the box, I connect it to a small dedicated network where my PC is set for 192.168.1.x and run the Rockwell BOOTP/DHCP Server Utility, and browse with RSLinx Classic to see if a commonly-configured address shows up.
There are more sophisticated ways to learn the IP address (do you know how to use Wireshark ?), or we can walk you through the COM port settings of your serial port and the simple null-modem serial connection.