@mibvip 

Hi,

Broadband modems themselves also have a local network.

When occasionally no connection to the ISP is available the modem might momentarily assign a local IP address to the device that is connected to it.

In your case the modem is likely using the local subnet 192.168.0.x, which is the same as the Archer A64 uses by default.

Having IP addresses of the same subnet on the WAN as well as the LAN breaks the network. There is no way a router could work under such a condition.

For that reason TP-Link decided to implement a logic in their routers that automatically changes the router's own LAN subnet to another one that doesn't conflict with the subnet it detects on the WAN port. (so the Archer A64 changes its LAN IP address from 192.168.0.1 to 192.168.1.1)

While this solves the actual network conflict and saves the day for the majority of (casual) users, it does cause trouble for users that have assigned static IP addresses within their LAN network or do for other reasons rely on a specific LAN subnet.

There are two ways to prevent the issue that you are seeing.

Either:
    1) Permanently change the LAN network of the ISP modem from 192.168.0.x to something else. (i.e. change the "0" to anything from 1 to 254, like 192.168.1.x, 192.168.9.x, 192.168.234.x)

Or (if 1 is not possible):
    2) Permanently change the LAN network of the Archer A64 from 192.168.0.x to something else. (i.e. change the "0" to anything from 1 to 254, like 192.168.1.x, 192.168.9.x, 192.168.234.x)
 

  @woozle 

Thanks for the quick response!

Unfortunately i am 300km away right now and i have remotely access only to the primary router. So i can't change the LAN network of the ISP modem.

Also i have no access to the 2nd router to change it's settings if i change LAN ip of Primary router.. Forgot to open "Remote management", but i was thinking that port forwarding of port 80 (i checked it's listening on this port) via Primary router will give me access to the second, but for some reason it's not working this way.

For now i found another response here for the almost same situation and there was advice to change DNS servers of the Primary router DHCP.

So i did it with public dns servers and now more than 11 hours i still have no problem, but it's too early to say if that fixed the problem..

I'll check everything when I'm there and keep your advice in mind.

Edit: Also i have Pi-4 there and i have SSH access. So i nmap 192.168.0.x and 192.168.1.x

There are result for my devices on 192.168.0.x, but there are also results for 192.168.1.x:

Nmap scan report for 192.168.1.0
Host is up (0.0072s latency).
Nmap scan report for 192.168.1.19
Host is up (0.011s latency).
Nmap scan report for 192.168.1.255
Host is up (0.0078s latency).

So i am not sure if actually the broadband modem's network is the reason cuz from this results i think it uses 192.168.1.0 which is okay.