The modern Smart Grid (SG) requires the adequate exploitation of Ancillary Services (AS) in order to dynamically support evolving energy demands, ensure grid stability and optimize energy trading transactions. Distributed Renewable Energy Sources (DRES) are considered as the most promising avenue to underpin the dynamic composition of AS. Nonetheless, the inherent legacy, distributed and resource-constrained properties of DRES deployments trigger a plethora of challenges with respect to the underlying end-to-end (E2E) data communication performance with direct implications on AS orchestration. Hence, assuring resilient and scalable E2E AS provisioning is a highly challenging task requiring advanced networking mechanisms. This work goes beyond architectural, policy-level and theoretical suggestions of how 5G can be utilized in the SG context and provide a proof-of-concept, system for DRES management. We introduce Ukko; an open-source, 5G network service design facilitating the programmable, "in-network" orchestration of DRES management, supporting real-time AS application requirements. Experiments conducted over a UK-wide 5G testbed using real use-case scenarios demonstrate that the proposed solution can assure scalable and resilient DRES management at the likely occurrence of data communication challenges.