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The basic idea of this work is to achieve the observed relic density of anon-thermal dark matter(DM) and its connection with Cosmic Microwave Background(CMB) via additional relativistic degrees of freedom which are simultaneouslygenerated during the period $T_{\rm BBN}~{\rm to}~T_{\rm CMB}$ from along-lived dark sector particle. To realize this phenomena we minimally extendthe type-I seesaw scenario with a Dirac fermion singlet($\chi$) and a complexscalar singlet ($\varphi$) which transform non-trivially under an unbrokensymmetry $\mathcal{Z}_3$. $\chi$ being the lightest particle in the dark sectoracts as a stable dark matter candidate while the next to lightest state$\varphi$ operates like a long lived dark scalar particle. The initial densityof $\varphi$ can be thermally produced through either self-interacting numberchanging processes ($3 \varphi \to 2 \varphi$) within dark sector or thestandard annihilation to SM particles ($2 \varphi \to 2~ {\rm SM}$). The latetime (after neutrino decoupling) non-thermal decay of $\varphi$ can producedark matter in association with active neutrinos. The presence of extrarelativistic neutrino degrees of freedom at the time of CMB can have asignificant impact on $\Delta \rm N_{eff}$. Thus the precise measurement of$\Delta \rm N_{ eff}$ by current PLANCK 2018 collaboration and futureexperiments like SPT-3G and CMB-S4 can indirectly probe this non-thermal darkmatter scenario which is otherwise completely secluded due to its tiny couplingwith the standard model.

CMB signature of non-thermal Dark Matter produced from self-interacting dark sector