Why iOS cannot preview DWG on its own
Quick Look handles PDF, images, and Apple's own document types. DWG is a proprietary AutoCAD container, so nothing in iOS knows how to draw it and the file shows as a blank document.
That is why almost every “DWG viewer” in the App Store is really a client for a conversion server. The file goes up, a raster or PDF comes back, and the drawing has left your control.
Site work without a signal
Basements, plant rooms, steel-framed buildings and rural sites are exactly where you need the drawing and exactly where the network fails. A converter-backed viewer is useless there; a local parser is not.
Because CadRoam keeps opened drawings in a private on-device library, a file you imported at the office is still readable at the site with the phone in airplane mode.
What is supported, precisely
CadRoam reads model-space geometry, blocks and layers from drawings saved as AutoCAD R13 through R2018 and later. A file whose signature falls outside that range is reported as unsupported rather than displayed as an empty sheet.
Paper-space layouts, annotative scaling and exporter-specific records outside the supported scope may render partially. Where the engine skips a record, the app says so.