How about casting without INTERNET. You know the home (log, brick or mobile) with just a generator or solar, laptop, .5TB thumb drive (with lots of music and movies) and WiFi router. Some speakers don't have buttons to turn on blue tooth and that only accounts for one or two. Forget about the ones with AUX ports like the Google Max speakers. I get voice commands won't work but It all seems to turn into bricks. If you did have a cell signal it probably would not be any good for data but if it was and you you had a USB to Ethernet adapter for your phone to plug in your router you would burn through your "tethered" 10 GB in a couple of days. Is there a proven way to use smart speakers as wireless speakers without INTERNET that will work?
No unfortunately because they're just not designed to be used in that way if you're looking to use casting.
However, it should work as a normal Bluetooth speaker if you set it up beforehand: