Does the S55H support Dolby Atmos?

Yes. It decodes Dolby Atmos and DTS Virtual:X and virtualizes height effects through its front drivers. It has no upward-firing speakers, so the overhead effect is simulated rather than true.

Do I need the TCL app to use it?

No. The bar works over HDMI, optical, Bluetooth or AUX without the app. The app is used for AI Sonic room calibration and for adjusting sound modes.

Will the wireless subwoofer work with any TV?

Yes. The subwoofer pairs wirelessly to the bar, not to the TV, so any TV connected to the bar gets bass. It needs a power outlet near where you place it.

The sound bar, the wireless subwoofer, a remote, and the cables and documentation TCL includes for setup. Check the listing for the exact cable bundle.

Is it good for a large room?

It is rated up to 220 W and TCL positions it for apartments, bedrooms and small-to-medium rooms. In a large open space, a bar with more channels and rear speakers will fill the room better.

Can I add rear speakers later?

No. The S55H is a fixed 2.1 system with no expansion for rear surrounds. If you want true surround, look at a bar that supports optional rear speakers.