Win Server 2016, DNS/IP Trouble - Resolving public IPv6 Address - Destination Network unreachable

Hi,

I'm having a little trouble with my network here. I'm hosting 2 Windows Server 2016 instances, one at the HQ and one as a branch, some 60 km away from there. I have set up an Active Directory Domain, DHCP Servers and DNS Server successfully. Between the HQ and the branch office, I have set-up a persistent Dial-Up Connection with RRAS to connect the two networks. Communication via IPv4 works flawless, I can RemoteDesktop from HQ into the "offsite" Server.

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Now I also wanted to set-up IPv6 Support on both sites. I have given out static IP-addresses to both Servers and created an IPv6 DHCP Scope on both servers and added Static Routes in RRAS. The DNS gets updated properly with both local and public IPv6 addresses.

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Now to the problem: When pinging (for example) the branch server from the HQ, I get "Destination Network unreachable" - Tracert works fine for 7-8 hops before it aborts with the same error. DNS correctly resolves the FQDN to their public ipv6 addresses on both sides.

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It looks to me as if every device knew where to go with the public IPv6 Adress, but can't reach the destination. Why do the devices even try to go via the public address? I'd rather have them use the tunnel from the dial-up connection.

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Strange enough I just realized while writing this, that Remote Desktop works, using the FQDN. But things like AD Replication, DNS Managment Console, DHCP Managment Console, all don't work when using the FQDN.

Help? :)