Because the template is installed from testing, you need to enable the
qubes-testing repositories to get that version of the package.
Edit /etc/apt/sources.list.d/qubes-r4.list and uncomment the
buster-testing line you want to use.
apt update
apt install qubes-core-agent-networking
If you dont want to continue using testing, just comment that line again
and run 'apt update' again.