5g is higher frequency, lower wavelength. That means it gets better potential speeds when it does work, but it's going to have more difficulty penetrating barriers, reaching longer distances, do worse in inclement weather, and so on. This is also a fundamental limitation of trying to reach higher speeds with this form of tech, because frequency and wavelength are inversely proportional. The point of this being that 5g is inherently less reliable/robust than 4g, and so it's not too surprising that you'd need a booster, especially if 4g in an area is not the hottest.