While all modern OS support the compressed IPv6 notation for DNS (2a0f:fc80::), some routers require you to enter the fully expanded notation (2a0f:fc80:0000:0000:0000:0000:0000:0000).
Help us fix this for everyone by asking your router manufacturer to support the compressed IPv6 notation for DNS.

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Workaround

You can use this online tool to expand IPv6 addresses before entering them.