The MacBook Air (2017) does not support 5k. If you have a Mac that supports a 5k monitor, two DisplayPort cables are required to connect between the adapter and the monitor.
5K may be a hidden resolution in macOS, so you may need to unhide it.
1. Connect your 5k monitor with two DisplayPort cables. The monitor should appear, albeit with lower resolution than 5k.
2. Open System Preferences. Open Displays.
3. On the Displays window that appears on your 5k monitor, option-click Scaled. All the hidden resolutions will be displayed.
4. Select 5120x2880.