I don't know of any AVR or display that supports DV that doesn't also support HDR10. Looking at the Denon web site, I don't see DV listed as supported by your receiver. But it does state that it has full HDCP 2.2 and HDMI 2.0a support, so I believe it has HDR support without a problem.
(Note: Looks like the OP made a typo in the model name. It appears he's talking about the AVR-S910W, not S-901W.)
Actually, this receiver comes with standard "HDR10" HDR, and only got Dolby Vision as an upgrade in firmware this summer. I agree with atc89092 - HDR10 is a free standard. There are no licensing fees for its implementation, so it doesn't make any sense at all not to incorporate it in the receiver. Dolby Vision, on the other hand, does
cost the manufacturer a licensing fee. (It would also cost Roku, as well, which is no doubt why it hasn't been incorporated yet, anyway, in its offerings.)
Look at this AVSforum post for models in this series, including yours. AVSForum Thread On Your Model-Type Receiver
Scroll a little in the first post in the thread until you see the line headed by this:
/X1200W/X2200W/X3200W/X4200W/X6200W (bolding from me)
You'll see it says the following about ALL of the models in this section:
- HDMI 2.0a(HDR10)/HDCP 2.2 on "all" HDMI inputs, able to pass through 4k/60Hz/4:4:4 color sub-sampling (see post 7 for models with upscaling feature)
Now, go here to the Denon Firmware site: DenonAVR Firmware Updates
If you scroll down the page, you'll see that there was a June 2018 release for your model that specifically adds Dolby Vision capabilities. To get the firmware download, select your model at the top of the page and add your model's serial number. That should take you to the firmware download page for this AVR where you would get the Dolby Vision firmware update. Of course, follow the instructions on how to update.
Now, that's not going to help you with the Roku, of course, as the Roku doesn't offer Dolby Vision capability. But it absolutely supports HDR10, which your AVR had out of the box. If you have the correct Netflix subscription (You need the "4 Screen" plan), you can get Netflix HDR output on the shows that support it. Test one of the Marvel series, like Jessica Jones, Luke Cage, Iron Fist or Daredevil. All of those are available in HDR.