[KaiserPro]

> 2.4Ghz propagation is very reduced by obstacles like buildings

I never did much 2.4ghz stuff because that was what rich people did, or people mad enough to modify microwave oven magnetrons. However I was always under the impression that freespace loss on 2.4 was terrible. but it turns out its "only" ~9db more than 865

[Gibbon1]

Worth mentioning that 2.4GHz has a lot more attenuation due to clutter than 900MHz. Your problem is usually buildings and non line of site transmission paths. When the signal has to pass through and bounce off things your link budget takes a big hit.

There is the idea of the path loss exponent. In a vacuum it's 2.0. 900Mhz with clutter it's -2.5 At 2.4 MHz it's -3 and -5.8 it's -3.5.

Other downside for higher spreading factor spectrum is data rate drops which results in longer packets. Longer packets means more energy per packet and a higher chance someone else will blow your packet out of the water.

You've been able to buy 900 and 2.4GHz transceivers for the last 20 years.