If none of the above worked you might want to try to reset network settings. To reset your Network Settings tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.
The app uses Apple's Bonjour protocol to automatically detect Mac computers on your network. This is what you need to do for this to happen:
1. Run "Remote for Mac"
Download, Install and Run "Remote for Mac" app on your Mac
2. Check network
Make sure you are connected to the same network on your Mac and iPhone/iPad
If automatic detection doesn't work it indicates that Bonjour protocol might be blocked in your router, or by your network administrator. Also, some public networks might block incoming connections between connected devices, which will block your iPhone/iPad from connecting to your Mac. First, you would want to check if you able to connect using QR code (which includes your Mac encoded IP addresses):
1. Show QR Code
Click on the "Remote for Mac" icon in the menu bar, and select "Show QR Code". If you have more than one IP address, select "Show All" if you unsure which one is the IP address of your network.
2. Scan QR Code
Scan the code on your display using iOS app, by selecting "Scan QR Code" from the main menu on your iPhone/iPad.
In order to prevent unauthorized access to your Mac, the app uses HTTPS connection and authorization mechanism you can enable in the app settings. When enabled for every new device that tries to connect to your Mac - authorization dialog will be presented where you will be able to choose if to allow or deny access:
Changing authorization status
You can change per device authorization status later on using app Preferences > Devices: