Why are IPV4 sites not working and IPV6 sites work?

I am using a GPON ONT router as part of Airtel Xtreme fiber plan. We have a range extender connected to it (TP-Link TL-WA850RE). I believe this is messing up my DNS server. If the range extender is on, sometimes (not always) certain sites stop working. Only sites like google, youtube, facebook would work but others won't. Right now, I have four devices connected to the main router, 2 Windows, 1 Mac and 1 Android. The two Windows devices cannot access Stack Overflow but others can. Not sure, if it is a Windows related issue, might have faced this on Mac as well but not entirely sure. This issue

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