If your XS Max has been updated to iOS 15.4 or higher LTE is the ONLY network option. That’s why there is no way to turn it on; it is already on. AT&T, Sprint and T-Mobile now require all phones on their network to use only LTE for data and VoLTE for voice, unless the phone supports 5G. Then you can switch between LTE and 5G. Verizon will follow later this year. If you have good Wi-Fi you can also enable Wi-Fi calling by contacting your carrier. If you already have Wi-Fi calling but your Wi-Fi signal is weak that could account for your problem; try turning off Wi-Fi calling.