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We describe the challenges and pathways when probing inflaton as dark matter with the stochastic gravitational waves (GWs) signal generated during the (p)reheating. Such scenarios are of utmost interest when no other interaction between the visible and dark sectors is present, therefore having no other detectability prospects. We consider the remnant energy in the coherently oscillating inflaton's zeroth mode to contribute to the observed relic dark matter density in the Universe. To fully capture the nonlinear dynamics and the effects of back-reactions during the oscillation, we resort to full nonlinear lattice simulation with pseudo-spectral methods to eliminate the differencing noises. We investigate for models whose behavior during the reheating era is of $m_Φ^2Φ^2$ type and find the typical primordial stochastic GWs backgrounds spectrum from scatterings among highly populated inflaton modes behaving like matter. We comment on the challenges of constructing such viable inflationary models such that the inflaton will account for the total dark matter of the universe while the produced GWs are within the future GWs detectors such as BBO, DECIGO, PTA, AION-MAGIS, and CE. We also describe the necessary modifications to the standard perturbative reheating scenario to prevent the depletion of residual inflaton energy via perturbative decay.