Personally, I bought a really cheap camera on a stand, which allows me to capture my writing on my own little whiteboard (or on a piece of paper). A second option is to buy a cheap Wacom tablet knockoff that connects to your computer (in case the platform supports a shared whiteboard, or in case the platform allows the candidate to share their screen).

Eventually, I've even stopped using my camera. Following the lead of [this developer][1], I've learned to draw most of my diagrams using ascii text. This takes a lot of [practice][2], but it's definitely possible. 

But in your case, since you're the interviewer, when you see that an interviewee is struggling, you could just tell the interviewee that he can take a few minutes to draw the diagram on a piece of paper (even if you can't see the piece of paper yourself). This is not ideal, but it's better than nothing. Drawing things out can indeed be extremely useful in solving these types of problem. 


  [1]: https://www.youtube.com/c/EChanTech
  [2]: https://www.pramp.com/