dynamicdns: fails to detect NAT properly with multiple default routes

Created by: SunilMohanAdapa

With multiple default routes configured, when trying to save Dynamic DNS configuration, we get the following error:

Device "eth0
ens9" does not exist.

This is due to running the following command to detect the default routes on the system:

    default_interface=$(ip route |grep default |awk '{print $5}')