@Solla-topee 

I'm from Bulgaria.

The downgrade has been postponed.

I apply a solution: I connect a gigabit 5 port switch and use it with a port occupied by TP-Link.

I'm a little annoyed because the Wi-Fi ports are unusable, but it works.

Today is the first day without dropping off the internet in weeks.

I will continue testing, but the problem is very strange.

My problems with the router countinue.

I have a new problem. After some time work maybe in hot time (here is around 30C). The router stop DHCP service, the computers lost thier IP addresses.

I try to set static address to see in log of router, but the router is unaccessible!

At last I downgraded router to previous firmware - c7v5_us-eu-ca-jp-tw-up-ver1-0-4-P1[20180425-rel72768].bin

I do not have more ideas and If I will have more problems I could return this product in warranty period.