Check the connection of Ethernet cable to the device and PC. Check if the device is loading. The words “RL-C21” should appear on the screen, and if you hold the “Stop” button for more than 3 seconds, it will display the information about the IP-address, serial number and the subnet mask. For a detailed step-by-step guide, consult this Knowledge article.
If the device IP-address is 0.0.0.0, and there is a [D] symbol next to the IP-address, hold the "Stop" button for about 1 minute to reboot the controller and reset factory settings.
Another thing to check in case TestUp does not see the device is whether Windows Firewall is enabled and whether there is an antivirus installed. They should be disabled.