How to solve the problem with the congestion of the channel?

Question

Computer networks

There is such a system:
There are routers that distribute Wi-Fi in the institution. Authorization occurs through the captive-portal. All routers are located in different geolocations, roughly speaking throughout the city. There is a problem of channel congestion, since the data from each point is first written to the radius, and then to the server itself. If we conditionally take a channel width of 10 Mbps, and each router eats 2 Mbps, then there is a limit on the number of routers that can be installed without buying more servers, which will not be economically feasible. And the channel is halved as the data then goes back to the server to the router. Question: how to unload the channel without buying more servers?