We added a feature to Coastal Explorer on Windows a few years ago that lets you create a customizable anchor mode/alarm using the Screens browser.
To do that, open the Screens browser, select an unused screen, add a chart panel (or what ever panel(s) you want to appear when anchored), then click Options at the top of the screen. In the window that appears, give this screen a name like “Anchor Alarm” and pick any other options you want. In particular, check “Add a boundary circle alarm at boat’s position” and set the desired radius.
When you’ve anchored, select the Anchor Alarm screen either by using the Screens browser or pressing the number let corresponding to the screen.