The 5G technology is still developing and Apple does not use it yet, perhaps next fall. They are typically late in adopting the new standards preferring to let them mature before adopting.
The current 5G chipsets are also reported to have heat issues:
5G heat issuesI use wi-fi for data most of the time and have never noticed 4G bandwidth issues. I have a old 3G booster and had no problem streaming a 2 hour movie through Amazon Prime while camping using its HSPA+ through my phone boosted as a hotspot to a laptop.
Sure, if I was using Android I'd buy a 5G phone now.