Adding to this...
I was able to test Atmos on LG OLED Netflix via eARC, but not on Virgin media direct HDMI to AVR.
It occurs to me that the streaming version of Atmos might actually be contained in a DD+ stream:
Dolby Digital Plus (E-AC-3) provides up to twice the efficiency of Dolby Digital while adding new features like 7.1-ch audio, support for descriptive video services, and support for Dolby Atmos. Dolby Digital Plus is widely used by streaming and broadcast services to deliver surround sound audio...
developer.dolby.com
If so, this might cause problems for automatic switching between DD+ and the streaming version of Atmos (I.E. the AVR tries to use the same sound mode for both):
1) If you force the AVR into DD+ mode it won't automatically switch to Atmos.
2) If you select "Atmos + DSur" mode, Atmos streams will play in Atmos, but DD+ streams will be output in "DD+ + DSur".
This is consistent with what I found when testing with Netflix via eARC.
Note: This stuff is probably the same for all recent Denon / Marantz units.