One has to be SPECIFIC about the frequencies when discussing 5G.
5G is a technology, or to be more accurate, a mixture of technologies. It can, and is transmitted at frequencies [bandwidths] previously occupied by earlier technologies.
Therefore the common mistake to automatically assume that 5G is millimeter waves is wrong.
Millimeter waves do help to speed things up somewhat, but do not have the range or penetration power of the lower frequency technologies.
5G in the mid band [3.5GHz centimeter waves] will penetrate deeper into body tissue whereas 5G, band [26-28GHz, and above, mm waves ] will not penetrate much deeper than the skin. However, we do have concerns about millimeter waves and the skin:
Mid- and low-bands for coverage and capacity
Low-bands (e.g. sub-1 GHz) support widespread coverage, including indoors, across urban, suburban and rural areas, while mid-band spectrum offer a good mixture of coverage and capacity benefits.
The majority of commercial 5G networks are relying on spectrum in the 3.5 GHz range (3.3 GHz-4.2 GHz). This momentum makes it vital that regulators assign as much contiguous 5G spectrum as possible in the range.
In the long term, more spectrum will be needed to maintain 5G quality of service and meet growing demand. This includes more spectrum in the 3.5 GHz, 4.8 GHz, 6 GHz and 10 GHz ranges which will all be considered at WRC-23.
GSMA – 5G spectrum in the millimeter waves
Pioneering ultra-high speeds and the lowest latencies are dependent on access to spectrum in the latter range. Real-world networks have already shown gigabit speeds are possible, and vendor trials are hinging at an even greater potential.
For 26 GHz and 40 GHz, the global identifications WRC-19 give the bands a big boost. Regulators can now assign mmWave spectrum knowing there will be an ecosystem in place. The 28 GHz is already used in commercial networks, with a growing number of devices.
Source: The GSMA at WRC-23 https://www.gsma.com/spectrum/wrc-series/