Is there anything changed with this router?

Make sure you do connect to the main Wi-Fi.

Have you tried with another smart phone?

your current firmware version and hardware version of Archer C9?

Firmware version is 1.3.0, build 20161018, rel. 71163 Hardware version 3.0 I'm connected to the main 5GHz WiFi Nothing has changed with the router as far as I'm aware. I'll try from another phone & report back. Thanks

Ok, I installed Tether on another Android phone and was able to successfully edit clients. Went back to my own phone but am still unable to make changes, not did the changes I made on the other phone appear to have stuck. All I'm doing is changing the client type icon and the device name for better identification. I have reset the router to factory defaults, uninstalled & reinstalled Tether on my phone but nothing helps. I have previously been able to make edits on my phone but I'm not sure when it changed.

What's the model number and OS version of the Andorid phone?

Looks like you need to check the phone itself since other phone can successfully edit it.

Did you install any new apps on this phone recently?

It's a Xiaomi Mi Note 2 running Android 7. I've tried uninstalling and reinstalling the app but it didn't help. I recently did a factory reset on the phone, not sure if that's when the problem started.

Solved it! Turns out that uninstalling did not delete app data even though it warns you that it does. I cleared all data in settings & now it's working. Thanks for your help