If my guess above is correct, you need to do this.
1. Turn on remote & Spark.
2. Press and hold the button on Spark's
battery for about 3 secs until you hear a beep. The front LEDs on the Spark should be red and blinking.
3. Press and hold the pause, Fn, and Control button on the remote. It will start beeping. When it manages to pair itself to the Spark, the front LEDs will be green and the small LED on the remote will also be green.
Once you have paired the RC and Spark, connect your phone to the RC via WiFi.
The WiFi SSID will be something like Spark-RCxxxxxx and the password will be 12341234 by default.
Launch DJIGo4 app. Take-off like you have done before and see if the RC is working now.