Being able to ping both ways in a DOS CMD window is the first step. Without that, TwinCAT will not be able to find the CX. Even if you can ping, I generally have to disable firewalls for TwinCAT to find the CX in a broadcast search, best I recall. Since our PC's are off-web, no firewall is not a security concern.
Re RJ45 ports, when I get a new CX first thing I do is find which NIC port has EtherCAT drivers installed. In View Network Connection in Windows on the CX, you will see several NIC's listed. One NIC is the internal Ebus to hard-coupled I/O modules and may not have an RJ45 port. Look at the properties of each. One will have EtherCAT setup. I name that NIC "EtherCAT link" in Windows. To find which port it goes to, use a cable to another PC. You will see it change to "unplugged" when you disconnect the cable. I label that RJ45 port "ECAT" on the front. The other NIC and port I name "HMI link" and label "LAN", which you use to your HMI/Development PC. CX's vary. Some have one NIC split to both RJ45 ports. Long ago, with a CX1000 (like that), I was using the RJ45 for RT-Ethernet link to a BK9000 and needed another NIC for the HMI PC. I had to use a USB-Ethernet adapter, which is a pain since it needed to be unplugged and re-plugged to be seen after reboot (power outage on graveyard shift). The CX5020 (Atom) is better since the 2 RJ45 come from different NIC's.
