C+27 Alt 04:00 - 23rd Century
Logans Run. In this Alt, The AI's running the Arcologies were much simpler and much less tolerant of human variation. They locked down their arcologies and their "Sandmen" enforced an ironfisted rule. Well, more silicon fisted.
Recently, a Sandman named Logan went rogue and has discovered that the outside is livable. He escaped and is running around with Jessica, an Android named REM, They are searching for "Sanctuary", completely unaware that the myth of Sanctuary actually describes the arcology project. He is pursued by Francis and other Sandmen. Logan destroyed the AI running New St. Louis. The other AIs send agents and a new AI to recover control. The new AI is built with new technology and is considerably more advanced than the old timers running the other arcologies.
There are a lot of randomized survivor communities and light contact with other worlds. But this Earth is an isolated, low tech backwater.
The biggest and most advanced free range civilizations are in Africa. From the point of view of off-worlders, the Capital of Earth is Lagos, Nigeria, Which is running at 1914 levels with bits of Galactic technology.
The Gate is, as always at Mammoth Caves. Currently the caves are inhabited by "Indians", They'll be suspicious, but willing to talk and trade. Among their artifacts are objects looted from the ruins of Towns and cities. The population within 100 miles of the Caves is about 10,000 people
What used to be Nashville and Louisville were nuked during the Great War in 1988. The war and 200+ years didn't leave much to loot, what there is has been picked over pretty thoroughly, but enough is left to be suggestive.
Although most of the world struggles in low tech poverty, the Domes are "Modern" to outside eyes. They do occasionally send agents to Lagos or Vera Cruz to acquire samples of technology for examination.