Most mobile data is consumed indoors, and indoors is where networks perform worst. The reason is a direct physical trade between how much data a frequency carries and how far it penetrates.
Higher frequencies carry more and travel less
Wider blocks of spectrum are available at higher frequencies, and wider blocks allow higher data rates. This is why new network generations move upward in frequency.
Higher frequency signals are absorbed more readily by materials and diffract less around obstacles, so they weaken faster with distance and stop at barriers that lower frequencies pass through.
The same property that delivers the headline speeds therefore limits where those speeds can be achieved, and the trade cannot be engineered away.
Buildings are effective shields
Concrete, brick and particularly metal structures attenuate signals substantially. Losses through an exterior wall can be large enough to remove a usable connection entirely.
Energy-efficient glazing is a common surprise, because the metallic coating that reflects heat also reflects radio signals, making modern buildings harder to serve than older ones.
Interior walls, lift shafts and floors add further loss, so coverage at a window can be good while coverage in a central room is unusable.
Adding towers outside has limits
Operators can increase transmit power or add sites, but the return diminishes because building loss applies regardless of how strong the outside signal is.
The uplink constrains it further, since a phone has far less transmit power than a tower and must be heard back through the same walls.
Serving indoors means being indoors
Distributed antenna systems and small cells placed inside buildings solve the problem by removing the wall from the path entirely.
These require installation, power and a connection back to the network, which makes them a building project rather than a network one, and someone has to fund it.
That funding question is why large venues are well served and ordinary offices and homes frequently are not.
Offloading is the practical answer
Wi-Fi already has infrastructure inside almost every building, and calling over Wi-Fi lets a phone use it for voice as well as data.
This is why indoor mobile experience often depends on the local network rather than on the mobile operator, despite the phone continuing to display a mobile connection.
The structural conclusion is that indoor coverage is largely a property of the building, and network generation matters less inside than the materials of the walls.