T-Mobile, AT&T, and Verizon each provide 5G deployed across the Loop, River North, the Magnificent Mile, and major Near North and Near South Side corridors. The Loop's main commercial streets, Millennium Park, Navy Pier, and the Museum Campus on the lakefront all have strong outdoor 5G.
The Chicago Riverwalk has reliable 4G throughout. Lincoln Park, Wicker Park, Logan Square, and Pilsen are all well-covered. The CTA's elevated L train system (the El) has 4G at outdoor and elevated stations. Underground stations on the Blue and Red Lines have variable coverage on platforms.
Some tunnel sections lose signal entirely. O'Hare International Airport has full 5G coverage. The North Shore suburbs including Evanston are covered by the same networks. Suburban areas along the Metra commuter rail corridors have 4G at stations.
The lake can lose signal quickly offshore, so keep that in mind for architecture boat tours.