That’s a good question; apologies I missed it. Firmware 3.216 is the latest stable release for the Mango. I’d make sure that’s the case on your device. Beta is beta for a reason, after all.
Here’s a ‘known good’ WG Server using a Flint (AX1800, f/w 4.2.1-release5) to a WG Client of a Certa (AR750, f/w 3.216). The Certa’s difference between your Mango is really just the difference of the Wi-Fi-AC radio. This config is as simple as it gets. I’m confident you can template from it as you need: