Why is wifi not working properly on my android smartphone (xiaomi mi3)?

Question

Android

Hello.
I recently purchased a xiaomi mi3 smartphone.
From the very beginning of working with him, there were problems with wifi.
Most often, the smartphone cannot connect to wifi. (More details. Having turned on wifi in the settings, the smartphone starts searching for networks. My network often simply cannot be found, although I am sitting a meter from the router. When the network is still found, I try to connect. But I can’t connect. Under ssid, the inscription "Saved. WPA / WPA2 protection". Anyhow, when I try to connect, my point simply disappears and is not displayed in the list. In general, the list of available access points behaves very chaotically. A smartphone, being in one place, can show 5 found points, and after 3 seconds none.)
Sometimes restarting the smartphone helps. And everything is connected in the regular mode, and very quickly. But after 20min-1h the network breaks. When you try to reconnect, the problem described above occurs. A couple of days ago, the smartphone was able to work for 3 days without such problems. Although the settings of the router and smartphone did not change.
What have I tried to fix this problem?
1. Rebooted the router (by the way, my D-link DIR 300 router)
2. Changed the Wireless Channel from 6 to 11
3. Changed the firmware on the smartphone, and on all three firmwares that I had, the problem remained. (currently firmware 5.5.15 from miui.su)
Before this smartphone, there were 3 android smartphones, and everything worked fine with my router. My opinion is that the problem is in the wifi of the device itself, because I tried to connect to a network created by another smartphone, and it was not possible to connect (the same problem is "Saved. WPA / WPA2 protection")
Sorry for the many letters, I wanted to describe the problem in as much detail as possible to make it clearer.
Below are screenshots of the router settings and screenshots of the wifi connection process.

I will be grateful to you for any advice. Thanks in advance.

Answer 1

Try changing the network mode. On the router.

Answer 2

Maybe there are still problems in the modem firmware on the smart itself. You can try to flash it, maybe it will come up.