Is the password defaulting back to admin?

What is the firmware build and hardware version you are on for the C9?

Do you know if you have had power outages?

No. Password changes to something else. I have tried lots of different combinations but it never logs me in. I have to reset the router. It then works for about 5 minutes and after that I can no longer log in. C1900 is V2 and Firmware is 1.0.0 (Nov 2016). Router sits on a UPS.

What would be best to do is to do a factory default on the router (hold the reset for 15 seconds). Once the router comes back online, log in and change the password to something else other then admin something more complex. Close the browser, leave it for a couple of minutes then try to log back in. If the login does not work, try a different browser to make sure the one you have been using is not caching anything.

Make sure you are not disabling Local Adminstration under Advanced > System Tools > Administration. 

One last thing would be that if you are binding your router to your TP-Link ID, your email and password for that TP-Link ID would be the login, not admin anymore.

Here is a article that talk about how to bind it for reference: https://www.tp-link.com/us/faq-1366.html

Thanks for the update. Glad to see that the device is running as expected now.

Might have been something that got in the way that caused something in the firmware to become abnormal.