Currently, the variable ROUTES belongs to the VM context and not to the network interface variable.

Custom routes are never read because of https://github.com/OpenNebula/one-apps/blob/master/context-linux/src/etc/one-context.d/loc-10-network.d/functions#L536

export static_routes=$(get_iface_var "$1" "ROUTES")

The quickest solution is changing the line for

static_routes=${ROUTES}

This sets up the custom routes correctly