You will not be able to do this with your current setup. The TV will decode the TrueHD Atmos stream to DD and strip the Atmos meta data, then pass that via ARC to the soundbar. The demo vids you have are DD+ and can be passed via ARC to the soundbar, keeping Atmos.
Your only solution would be to get an external media player, like a Shield, and connect it to the soundbar directly. This is assuming the soundbar can decode TrueHD and can then pass 4k HDR (if you have UHD rips) to the tv.
Vizio P55-C1 | Denon AVR-X1200W
 DefTech ProMonitor 1000s | ProCenter 2000 | SR-8040BPs [.1] SVS PB1000 [.2] Micca R-8C
Nvidia Shield | Zidoo X9S | Popcorn Hour VTEN