Mythological Underworlds/Otherworlds or gates to such places:
Avernus
Mictlan
Niflheim
Tartarus
Xibalba
Buddhist “Hell” equivalents:
Arbuda
Avici
Hahava
Naraka
Nirarbuda
Raurava
Utpala
Words with connotations of hell you can mix it up with.
Abaddon
Abyss
Aether
Dis
Gehenna
Miasma
Makai
Nether
Ordeal
Pandemonium
Perdition
Pestilence
Purgatory
I quite like the sound of Tartarus. It doesn’t sound like a silly name for a modern city and the reference isn’t cheesy sounding when you know what it is. I also wrote Miasmapolis, but stared at it a bit and deleted it – then wrote it again to tell you all my silly name.
Or you could go the opposite route and name it somewhere really pleasant sounding. Like Clearwater or Haven, whoa -- juxtaposition, it’s like poetry.