Does changing the WAN DNS servers cache and purge resolvers?

I have recently configured a Pepwave Surf SOHO MK3 device that is running firmware version 8.0.0 build 1429. I have observed when the WAN DNS servers are changed, it results in varying behaviors with clients.

For example, if the WAN DNS servers are configured to resolve to, the laptops and PCs can access the internet with no issues. Smart devices such as smart TVs cannot.

When the WAN DNS servers are updated to resolve to, the laptops and PCs continue being able to access the internet. Smart devices can now access the internet.

When the WAN DNS servers are reverted back to, the laptops, PCs and smart devices can continue to access the internet.

That’s weird. Why not? Google DNS is used by everything. Don’t see any reason why the smart TVs wouldn’t work. If it was me I’d go back to this step and do a network capture and work out why the smart TVs are unhappy when they should be fine.

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They do work once the WAN DNS servers change from the configured IP addresses. That’s the strange behavior. For example, if the router is configured with the IP addressed, they fail.

Once the WAN DNS servers are updated, they function again. It’s independent of the DNS servers. It suggests that the previous DNS records are cached and the devices are attempting to connect however fail. The change in the WAN DNS server setting seems to reset it.