How does Unbound resolve DNS?

[Karl]

Hi,

I use OPNsense with two WAN interfaces (DSL and cable) and two internal nets (LAN+WLAN). I run Unbound to resolve DNS queries for my two local networks. I got the problem that when the cable WAN was down, DNS did not work anymore for my two internal networks. I expected Unbound to resolve DNS via the still working DSL WAN.
How does Unbound resolve DNS queries?
If "Enable Forwarding Mode" is enabled, it uses the DNS servers configured under "System->Settings->General". If there are two DNS servers configured, 1 for DSL, 1 for cable, Unbound would automatically use the other one if one of them was down?
If "Enable Forwarding Mode" is disabled, which WAN connection would Unbound use?