How about if each city had 1 emergency cellular site on top of the tallest building. An extremely hardened site. Intended just to handle calls to 911. And maybe to send out wireless emergency alerts also. Would that be doable somehow?
Not really.
Cell sites are not usually set up to cover wide areas. They are designed to cover their "cell".
Many landlords don't want generators, especially on roof tops, or in downtown areas where real estate is expensive. Battery systems are very heavy, and floor loading becomes an issue. In wilderness area, forests, etc. storing large amounts of fuel is an issue. Getting fuel to the site is a challenge. Solar can work, but it takes a lot of panels to be reliable, and usually an additional energy source just in case.
At common cellular frequencies, coverage over wide areas becomes a challenge.
We already have tools to do this. It is AM broadcast radio. FM broadcast radio. Broadcast TV stations, all using the EAS system.
Then we have NOAA alerts, which can handle EAS and other localized alerts.
People should have a battery powered AM/FM radio and ideally, a radio with NOAA capability. Not expensive. Most people have them in their car, but forget how to use them.
Some cities have alert sirens and/or audio alert systems which are a good resource.