If you want to open RMS-protected PDFs in Edge, it needs to turn on Internet Explorer mode.
To turn on Internet Explorer mode, use the following steps.
In the address bar for Microsoft Edge, type edge://settings/defaultbrowser and then click Enter.
Slide the Allow sites to be reloaded in Internet Explorer toggle to ON.
Restart Microsoft Edge.
Internet Explorer mode is now enabled. It needs to enable "Reload in Internet Explorer Mode" to open PDFs with Foxit PDF Editor.
Navigate to the website you want to view in Internet Explorer mode.
Click the three dots in the upper right corner of the browser window.
Select Reload in Internet Explorer Mode.
If you are on Microsoft Edge version 92 or earlier select More tools > Reload in Internet Explorer Mode
Then enable "Open this page in Compatibility view" and "Open this page in Internet Explorer mode next 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.