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#11
Does your router/firewall have a DNS relay built in?
If it does then your DNS should be set to the firewall not your ISP. You might have problems with both WAN and LAN functions aiming this beyond the firewall.
Since the firewall gateway is 192.168.20.1, that's probably where you should set your DNS as well.
I would also suggest that you disable (uncheck) IPV6 support in your network adaptors and test...