"Network not reliable" message

Is the iPad the only device having this issue?

•     Make sure that your Wi–Fi router's firmware is up to date. If you're using an AirPort base station or Time Capsule, follow these steps.
•     Make sure that all Wi–Fi devices you want to use support the settings this article recommends.
•     If possible, back up your Wi–Fi router's settings.
•     If necessary, refer to the product documentation or manufacturer's website.
•     Forget or remove the Wi-Fi settings for your network from any devices that connect to your Wi-Fi router. This will prevent the devices from attempting to connect to your network with the old configuration. You'll need to reconnect these devices to your network when you've finished applying the new settings.

Configure all Wi–Fi routers on the same network with the same settings. Otherwise, devices could have difficulty connecting to your network, or your network could become unreliable. On dual-band Wi–Fi routers, configure both bands to have the same settings unless otherwise noted below.

Please visit the apple troubleshooting page: https://support.apple.com/en-us/HT202068

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Are you able to connect to your extender network via another computer/device?