ahh now I got it. Your service is starting to early.
I guess you would need to add following line to /etc/wireguard/wg0.conf
PreUp = /boot/dietpi/func/obtain_network_details
pls add before PostUp section
In general this is fixed for new installation with current DietPi release. https://github.com/MichaIng/DietPi/issues/3519#issuecomment-626683268