If you want to open RMS-protected PDFs in Edge, you need to enable "Reload in Internet Explorer Mode" to open PDFs with Foxit PDF Editor.
Click Settings and More in the upper right corner, enable "Reload in Internet Explorer Mode".
Then enable "Open this page in Compatibility view" and "Open this page in Internet Explorer mode nexe time".
BTW: Microsoft Edge 83.0.478.37 or later can open protected files directly, but also requires "Reload in Internet Explorer Mode" to be enabled.