The issue is that the DLink DSL-502T router as shipped in large numbers by
Xtra has a flaw in the DNS resolver. If a DNS request for an AAAA record is
sent to the router, I've observed it sending back a malformed response
packed. T
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I presume this is probably firmware specific. Have you tested with various firmwares to confirm the problem?

That router has build chain available publicly I believe. While not going to solve the issue for a majority of users it should be easily fixable with a firmware update by those who want to fix it.