Does a DAC like this even exist?

I do not see "lock screen controls" in the roon-settings-setup menu. What does that do if I find it?

Maybe he is referring to the per-audio device volume control setting. Under Settings > Audio if you click the gear next to an enabled device you can disable volume control. But that probably isn't what you want, since you mentioned wanting to control the volume remotely.

But another option would be to set the per-audio device volume limit. If it keeps going all the way up which is a problem, you could try setting the limit to the max you are willing to use, and maybe that will be where it goes up to.

Still, it is very strange that you are running into a problem like this. I would try using a second Roon client, like a second iPad or computer or something, to see if it still happens. 

Lumin is a very good choice for a streaming DAC with remote volume control over the network, that will work with Roon, AirPlay, etc. Easy to use and works well with pretty much anything someone wants to do, plus top-notch hardware and sound quality and continuous features and improvements added through free firmware updates.

But I still think something is up because the problem you're describing doesn't really sound like something specifically due to Roon.

A cheap alternative is the new Wiim Amp ($300).  A pretty impressive product!

The Lumin Leedh volume control is almost perfect. The best control I have heard on a DAC or streamer. I did a shootout with my Benchmark LA4 preamp vs the Lumin X1.

The volume control on the LA4 is hard to beat by any gear. The Leedh matched it sonically until the very lowest volume. At that low range the Leedh no longer had consistent adjustments of the volume. Nitpicking. the Leedh is the only DAC I would use direct to amp if I were not to use the LA4.

I used ROON with the Lumin. I never had an issue with the volume. I used the streamer to stream into the Lumin X1 DAC (with fixed ROON volume) and also streamed out to via the Lumin SPDIF to control a PeachTree GAN1. The GAN1 has no volume control so I used the Leedh. I forgot if I also streamed to the GAN1 using variable ROON volume. Too many Lumin settings to remember now.

I think that a Bluesound Node, which is a streamer/DAC with BlueOS, which is easy and fun to use, will do what you need.

PS

Control of Bluesound NODE is via WIFI, which in my house extends beyond its walls.