If you make your material ‘Unlit’](Using the Emissive Material Input | Unreal Engine Documentation) and plug your diffuse into the emissive field, your mesh won’t be affected by light (and should still cast shadows)
If you make your material ‘Unlit’](Using the Emissive Material Input | Unreal Engine Documentation) and plug your diffuse into the emissive field, your mesh won’t be affected by light (and should still cast shadows)