Mobile ux: How to indicate the presence of side panel/drawer/sidebar

The company I work for has had a desktop website for ages, but recently I had the task to make a small mobile version with limited functionality. It's been online for a while now and I've been tracking how the people are using the mobile site, but it's gotten me concerned. On a page to generate a report, the user has to select a date and some other options, so I put it inside a side panel/drawer/however you want to call it. It appears when you click the button in the top right:

But most users don't seem to find it and just go to the desktop site after a few seconds.

Question:

Do any of you have any tips on how to communicate the presence of sidebar? I want to keep the ui clean with not too many distracting elements and popups to "explain" it are pretty much out of the question for me (I hate them myself).