The "5G" isn't really about any new way for ordinary bases to talk to mobiles, like 2G, 3G and 4G were. It's about integration of services, back office processing and backhaul.
Also spectrum above 2.6GHz is really only "WiFi" or Line of Sight.
The way to avoid not spots is licence conditions requiring 100% geographic coverage. No new technology needed. Otherwise operator cherry pick where to put bases so as to make a profit on the capital investment and running costs. Rural has higher running costs due to engineering travel time and backhaul costs.
This is also why performance can be poor. Not technology. There is no incentive to have x3 or x9 density base stations needed to increase speed and availability unless data is charged per megabyte used. With capped or unlimited subscription or PAYG, the only way you get speed past a certain point is enforced licence conditions.
There needs to be more fibre / real broadband so that Mobile only has actually mobile users.