1. Mobile networks do not operate across the entire spectrum, they operate on frequency bands. The bands are country and mobile operator specific. Then there is

1. Mobile networks do not operate across the entire spectrum, they operate on frequency bands. The bands are country and mobile operator specific. Then there is the modulation which might vary across each frequency band and can be can frequency division based (for rural areas with a low network capacity but a longer range) or time division based (a shorter range but a higher capacity). On top of that, there is the channel bandwidth that also varies depending on the spectrum channel utilisation. In some areas (or countries), the channel might already be utilised for something else, for example a national train network communication system or by military. Such spectrum channels can't be reallocated, so usually new, country specific, channels are allocated. A capable, laptop integrated modem would have to support them all. Some regional mobile operators have also extended their networks with non-standard frequencies that are locally important but are not supported outside the home turf (Australia's Telstra and Scandinavian mobile operators are prime examples. Telstra even has its own research division, and they have extended the spectrum with non-standard frequencies more than once. They have also licenced their technology to mobile operators overseas on a small scale, e.g. Hong Kong's CSL and to others). You can check the actual spectrum allocation for 4G and 5G at https://en.wikipedia.org/wiki/LTE_frequency_bands https://en.wikipedia.org/wiki/LTE_frequency_bands and https://en.m.wikipedia.org/wiki/5G_NR_frequency_bands https://en.m.wikipedia.org/wiki/5G_NR_frequency_bands, respectively. It varies from 410MHz to 5.9GHz for 4G and from 600MHz to 71GHz (!) for 5G NR with most 5G frequencies being upward of 2.6GHz – to avoid the interference with 4G. Most importantly, though, the radio wave interference plays the fundamental role. Whilst 4G and 5G might share parts of the spectrum on the paper (which varies wildly for each), they can't share the same channel at once even if the modulation is different. It is not possible to go against the laws of physics and magically have two different standards to share the same radio frequency. Electromagnetic waves will interfere when the channel utilisation goes up. Then there is the supply problem. There is exactly one commercial 5G modem vendor in the entire world. It is Qualcomm. And their modems are not known neither for the power efficiency nor for the low heat dissipation – they are both, power hungry and run stinking hot, when in use. 2. That remains to be seen. 6G is already under development and is undergoing the early testing. It is too far away from a world wide deployment, but it will be in the terahertz spectrum == incompatibility with existing technologies.