We present the validation of TOI-2407 b, a warm Neptune-sized planet with a radius of $4.26\pm 0.26$ R$_{\oplus }$, orbiting an early M-type star with a period of 2.7 d and an equilibrium temperature of $705\pm 12$ K. The planet was identified by Transiting Exoplanet Survey Satellite (TESS) photometry and validated in this work through multiwavelength ground-based follow-up observations. We include an observation with the novel complementary metal-oxide-semiconductor (CMOS)-based infrared instrument SPeculoos InfraRed Imager for Transits (SPIRIT) at the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS) Southern Observatory. The high-precision transit data enabled by CMOS detectors underscore their potential for improving the detection and characterization of exoplanets orbiting M-dwarfs, particularly in the infrared, where these stars emit most of their radiation. TOI-2407 b lies within the boundaries of the period–radius Neptune desert, an apparent scarcity of Neptune-sized planets at short orbits. Further characterization of TOI-2407 b, such as radial-velocity measurements, will refine its position within planetary demographic trends. This system also provides a comparison case for the well-studied Neptune-sized planet Gliese 436 b, of similar radius, period and stellar type. Comparison studies could aid the understanding of the formation and evolution of Neptune-like planets around M-dwarfs.