Will 4G signal booster work with 5G phone?
A 4G signal booster will definitely work with AT&T's 5Ge network (5G Evolution) because it is the same, only faster. 4G boosters can also technically work with 5G phones with a caveat: It will enhance signals only when you set the phone in its "settings" to operate on 4G / LTE. There are also instances when it will boost signals when 5G phone automatically falls back to 4G or LTE mode which will happen when 5G coverage is not detected.
Please note that 5G signals will not penetrate buildings as easily as 4G LTE signals currently do, due the intrinsic nature of the new 5G frequency waves. 4G LTE frequencies currently range from 600 MHz to 2700 MHz (or 2.7 GHz) whereas 5G is planned to be in 5 GHz to 46 GHz range which makes it far more difficult to be able to penetrate building materials.
A signal booster cannot boost signals when there is no signal - they require some signal to boost it. Therefore, a 4G booster will be an asset when a 5G phone does not detect any coverage indoors or in vehicles. Read more at our 4G & 5G and Everything 5G blog post.