Openwrt Serious BUG and solution: WiFi Restarts several times a day

I am using openwrt with mt7621+mt7613+mt7603 for long time but suddenly I got an issue that WiFi restarts several times a day and it takes 5-20 second to reappear SSID.

I have tried in my ways but no solution found but if I use mediatek SDK to build the firmware then this problem solves.

Now the problem is mtk-SDK needs ubuntu 16.04 (too old) and I have my custom application which requires latest openwrt with ubuntu 20.04.

I have tried to port the mtk-sdk driver to the latest openwrt but failed.

Can anyone please help ?

Thanks in advance.

Is this "Hi, I got black box HW, running a black box OS, and it doesn't work, help me with my black boxes!" kind of question?

Let's start with this:

Please copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall

What is the nature of this custom application? I assume it is running on OpenWrt? What does it do?

Do you have log data for this? Could it be related to DFS?