KeeneticOS 3.6 and higher have the ability to assign a static (permanent) IP address to Wi-Fi Extenders.
Connect to the web interface of the Main Router (Wi-Fi System controller). On the 'Members' tab in the list of Extenders included in the Wi-Fi System, click on the 'Edit' button of the Extender entry.
In the window that appears, you can change the name of the Extender and specify for it the static IP address from the home subnet of the Main Router.
In our example, the IP address for the Keenetic Air extender was changed from 192.168.1.104 to 192.168.1.204 on the Wi-Fi controller.
After that, the Extender's web interface will be available at the new IP address you specified.