The JBL Xtreme will not be supported by the JBL Portable app while using Apple devices with iOS 14 or later versions due to incompatibility. 
You can continue to use the speaker through Bluetooth or Aux connection and manual pairing, but cannot stereo pair with another Xtreme through the JBL Portable App.  
There is no plan to release a new Firmware currently as the JBL Xtreme has been discontinued.

If you still wish to use JBL Xtreme with the JBL Portable app, please connect it with Apple devices with iOS 13 or below, or connect it with Android devices.