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The basic idea of this work is to achieve the observed relic density of a non-thermal dark matter(DM) and its connection with Cosmic Microwave Background (CMB) via additional relativistic degrees of freedom which are simultaneously generated during the period T BBN to TCMB from a long-lived dark sector particle. To realize this phenomena we minimally extend the type-I seesaw scenario with a Dirac fermion singlet(χ) and a complex scalar singlet (φ) which transform non-trivially under an unbroken symmetry Z̶ 3. χ being the lightest particle in the dark sector acts as a stable dark matter candidate while the next to lightest state φ operates like a long lived dark scalar particle. The initial density of φ can be thermally produced through either self-interacting number changing processes (3φ ⟶ 2φ) within dark sector or the standard annihilation to SM particles (2φ ⟶ 2  SM). The late time (after neutrino decoupling) non-thermal decay of φ can produce dark matter in association with active neutrinos. The presence of extra relativistic neutrino degrees of freedom at the time of CMB can have a significant impact on ΔNeff. Thus the precise measurement of ΔNeff by current PLANCK 2018 collaboration and future experiments like SPT-3G and CMB-S4 can indirectly probe this non-thermal dark matter scenario which is otherwise completely secluded due to its tiny coupling with the standard model.