First things first. Please do not use such a long thread title. I see that you always put everything in the title itself. That's not necessary. Just give a proper short description as title and provide details in the post content.
Back to your question, It happens occasionally. It's a ISP related issue due to DNS caching.
Try to reboot the router. Turn off the router, wait for 30 secs, restart it.
You can try to flushdns cache manually. Use the below quoted commands in command prompt.
Try changing dns to Google or other public DNS.
If none of these works, assigning manual IP to devices will fix the issue
This usually get resolved automatically in sometime without any troubleshooting. Just use manual IP for some time and allow the DNS caching to reset from the server end.
My Mi router is connected to my Airtel's router and it is working. I can connect devices on it (need static IP) but it status page still shows this error.
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