@TP-Link 

Good day,

Yesterday I reserved IP addresses on the MR6400 for the Deco M4. Since then, I haven't noticed any problem. Hope this action solved my problem. I'll confirm it next week, if everything works well by then.
Thanks for the suggestions.

@TP-Link 

Unfortunately problem still occurs. As I wrote previously I reserved 2 IP addresses for both devices (problem still exists). When problem occurred I tried to do as you mentioned: "check what the internet IP address is under the advanced>IPv4", however I was not able to do that via mobile app because of connection issue. In that time I only check configuration on my router. I only found 1 device (secondary device) -a primary device was not on list. I have just tired 4th suggestion - I have reset both devices and I have swapped primary device with secondary device. I hope that it will help. 

1 - I bought Deco M4 on 24th of October 2020

@TP-Link 

I also have the same problem with 1 specific node of M4. While that Node plugged in, the temperature is quite hotter than the others and its connection is pretty bad, after setup the LED is White then awhile later change to RED then White again. The whole system speed is not satisfied, it's around 30mb meanwhile my ISP package is 86mb, the modern wifi speed from ISP is full speed at 86mb. It's really a bad products & experience...

@TP-Link 

Propably I tried everything from this list:
1. reserved 2 IP addresses 

2. swapped devices,

3. checked IP addresses during a problematic situation (I checked them on router admin page because I am not able to check it via mobile app because application informs me that it is not able to connect to Deco devices. I tried to ping those devices but they did not respod). 

In addition I upgrade also firmware of my router.

Unfortunatelly problem still occures. I notticed one more thing. Gennerraly problem occures betwean 1:00PM and 2:00PM UTC. During that problem I found in router logs such logs (a lot of them):

XX:XX:XX:XX:XX:XX - mac of main deco device

xxx.xxx.x.xxx - its reserved IP address

Do you have any other suggestions? Please help. 

Dear @TP-Link support,

In December I switched back Deco to Router mode. I noticed that at the beginning it worked more stable (router mode + reserved IP address for Deco Device). Problem still occurred but once a week. However in last week loosing Internet occurs really often. 

@TP-Link Today I cannot use it because a red light is all the time. Today I tried my last idea. I changed Ipv4 setting 'dynamic mode' to 'static mode' however it did not help. 

@TP-Link Please help me because in this moment I cannot use this product. In addition I cannot loose Internet connection in random moments because it looks really not professional when I speak with my coworkers or manager.