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.