How to I debug a DNS issue I'm getting at home?

Hello,

What is the best way to troubleshoot a frequent DNS issue we get at home. We use NextDNS via out OPNsense firewall and it's been great for over a year. But over the last month certain sites won't load, although we can ping them so they do resolve.

If we wait 5-10 mins things will be ok and it's only certain sites too most will work when this is happening.

On the logging section of NextDNS (on the portal) is sees the sites we are trying and doesn't say they are blocked either.

When this happens I pop onto another network at home which goes through the same firewall but doesn't use NextDNS for lookups and it all works.

Any advise would be great.

Thanks