TP link BE900 v1.6. Firmware Version: 1.1.4 Build 20241129 rel.31309(5553)

I bought a used router. Unfortunately, there are problems.

When changing some settings, a message appears: operation failed.
For example, the Wi-Fi settings are not visible. It is impossible to change the network name or password. These settings are not there.

Also, when connecting to the Internet, the status of this Internet is not visible.
In the Tether application, the menu for setting up WiFi freezes and does not open.
When authorizing, the message operation failed appears. And when navigating in the router's web interface, this message appears again in some parameters.

What I tried to do:

Reflashing the router to 3 different firmware versions(1.1.2 / 1.1.3 / 1.1.4)

Reflashing via press pin.

Reset settings. Factory reset.

I also switched the router to AP and easymesh mode. Returned back to normal use

Change of equipment. Three people tried to solve the problem on different devices. With different internet. The system administrator also tried to solve the problem

Result no. The message does not disappear...

Although the router distributes the Internet via Wi-Fi and a local network.

That is, it works somehow, but why not in full?

What to do? Help with advice. I really tried a bunch of options to solve the problem. So far without result.