@Ivaylo 

No, nothing changed :( 

I have uploaded my router configuration. 

@Ivaylo Of course I tried these.  I've actually tried everything, such as turning off Multiuser MIMO, disabling NAT boost. 
What else should i try?  Disable DHCP server?  Or enable Smart connect 2,4G/5G? 

Yeah, but nothing. Probably the XIAOMi MITV-Stick cannot handle this router properly. I turn it on and there is no internet for 2-3 minutes, WIFI connection is, but no internet, after that 2-3 min. there will be internet by itself. If I disconnect it from WIFI and then back immediately, that is also a solution. Just having so much fool around every time isn’t exactly comfortable. And what’s ridiculous is that everything worked perfectly with my old wifi router, TP-Link WR 841N. After switch on immediately WIFi with internet.  But with the other device, with XIAOMI MiTV Box is still everything fine. Though the two devices are exactly the same, one has UHD support and the other doesn’t

Xiaomi’s answer: ,,I would request you to check if the DNS is correct. Check whether there is an interference source of the same frequency near the Xiaomi Mi TV Stick, turn off all other WiFi receiving interrupts and Bluetooth, wireless network card, etc. may interfere with the appliance, and then restart the router.” Of course it doesn’t help

After that: ,,I would like to inform you that we have already guided you with all the possible troubleshooting from our end, so I would request you to please get in touch with your respective point of purchase for further assistance.

However, I will record this as your feedback and surely will share your valuable feedback with our concerned authorities so that they will work on the same in the future updates and none of our customers face any issues similar to yours."