If "insecure rejoin" is disabled, you'll need to manually reset the device and reconnect to the Hub.
Method 1: Put the Hub in join mode by tapping the plus icon > select the device category > select the device brand. Then while the Hub is searching for the device, power cycle the device (by unplugging it and plugging it back in or taking the batteries out and putting them back in). Though the app may say that no devices were discovered, it’s possible that the device actually rejoined the ZigBee network behind the scenes. Navigate to the My Devices page and test the device to see whether it is responsive.
Method 2: If the above method does not work, put the Hub in join mode and while the Hub is searching, perform the reset steps to reconnect the device.
Again, removing the device from the app before performing these steps is not necessary or recommended.