Dog Monitor has an option to turn on the light of the phone when the dog's room is too dark or when you want to get the dog's attention. It can be turned on while the video is active by tapping on the flashlight icon.
If you are missing the button could you please check, if your Dog Station has a functioning flash?
If the Dog Station doesn't have a flash, but the front camera is present, you can switch cameras within the app, and use the display as a source of the light instead of a flash.