Make sure you Command Module (CM) and Firing Modules (FM) are all on the same radio frequency (FR). You can manually change the FMs to match the radio frequency of the CM you’re trying to join. Go to MENU, scroll down to [RF Channel], press ENTER and you can change the RF channel by pressing the YES ⬆️ and NO ⬇️.
Next, on the CM go to MENU, press [3 JOIN] + ENTER. The Graphical User Interface now displays the CMs number. On the FM press MENU and scroll down to [JOIN MODE] and press ENTER. The FM should now display the same number featured on the CM you’re trying to join. Press YES + HOME on the FM, and press HOME on the CM. All mods will display CONNECTED.