The most likely cause is that the transmitter and receive are not set to the same system IDs. Start by confirming that all devices are part of the same system, with the same ID codes marked on the labels for everything you’re using together. RC4 Private System IDentities are the key to our industry-leading data security.

Are you using other devices in the show that are working? If so, that would confirm that the transmitter itself is working.

Start by restoring defaults on the receiver that is not connecting. This will put it back to system ID0, the default, and set the DMX channels to 1, 2, 3, and 4.  Here is the quick-start guide for the DMX4dim, see page 6:
https://rc4.info/wp-content/uploads/2019/01/RC4Magic_S3_DMX4dim_QSG.pdf

And here’s a video of the process:
https://rc4wireless.com/knowledge-base/dmx4dim-restore-defaults-factory-settings/

If you are not using other devices that are already working, or if none of your receivers are working, you should also restore the DMXio transmitter to factory defaults.  Here is a video of that process:
https://rc4wireless.com/knowledge-base/dmxio-restore-defaults-factory-settings/