The sub 1GHz, especially 800MHz and lower is cells that are too big for high capacity, high speed data. Only a cheap way to extend voice coverage at expense of capacity.
Above 2GHz, toward 3.5GHz radio spectrum becomes progressively more Line Of Sight. So 3.5GHz is only much good for roof top aerials or femto cells.
Above the 4.5GHz / 6GHz you are looking at only open plan offices pico-cells or air-point per room WiFi.
I've used 10GHz band terrestrial Fixed Wireless Broadband and both Ku Band (11GHz to 12GHz approx) and Ka Band (19GHz to 21GHz) gear, not just as a user, but both as RF Equipment design engineer and also evaluation of systems.
28GHz etc is fantasy outside of a room or open plan.
Very much in the media, stuff by Regulators etc is nonsense.
Mobile is viable between 900MHz and 2600MHz bands. The 2300MHz is the only useful new band in Europe.
Most of the existing 900, 1800 and 2100 spectrum is massively underutilised:
1) Cells too big
2) Split between multiple operators. Using one shared Infrastructure, or even "roaming" where an operator actually HAS coverage, would almost double capacity!
Regulators need to be forbidden to auction as this encourages weak licence conditions to make auction price go higher. Auctions are the enemy of efficient spectrum use.
Conventional badly applied theories of Competition benefiting Consumer damage Mobile performance and competition as it's NOT like making baked beans. Spectrum is too finite. They need to be only competing for customers by offering shorter contracts, better deals from the same wholesale properly regulated spectrum.
Subsidy of handsets by subscription is hidden hire purchase.
Also hidden is the overcharging on voice and text, subsidising data.
The regulators have totally messed up.
The FCC is messing it up for the rest of the world too, as does greed of royalty earners like Qualcomm wanting to sell new model chips for new bands. The proposal to have LTE on WiFi bands is driven by chip vendor greed and to an extent Mobile Operators (Femto cells without an expensive licence or bothering to co-ordinate channels).
700MHz and 600MHz are madness as the cell size can't be controlled and even larger than 800MHz which is poor for cell size.
You can't beat the laws of physics.