Electro-tunable metasurfaces have attracted much attention for the active control of incident light at the nanoscale by engineering sub-wavelength meta-atoms. In this Letter, for the first time, to the best of our knowledge, a grating-assisted dual-band metasurface for spatial light modulation is reported that can operate in two crucial telecommunication wavelength bands, i.e., C-band and O-band. The proposed device consists of a silicon-nitride nanograting on top of a silicon-indium-tin-oxide (ITO)-alumina-gold stack. Effective medium theory combined with a modal analysis is used to study the guided-mode resonance dips at 1.55 µm and 1.31 µm in the reflectance spectra. We leverage the epsilon-near-zero effect of ITO by applying an external bias voltage to introduce large modal loss, which leads to the disappearance of the resonance dips at those wavelengths. We obtain a high modulation depth of ∼22.3 dB at 1.55 µm and ∼19.5 dB at 1.31 µm with an applied bias of -4 V and -5 V, respectively. Thus, the proposed metasurface may help to realize dual-band active nanophotonic devices.