What you could try is to set up a wireguard server on the mango with the gui and compare in the lucy settings of the mango with the settings in your openwrt router.

Additionally, looking at the mango config and comparing it to the instructions on the page you linked, it also suggests to use 10.200.200.2/24 in the client app, while you have set 10.200.200.3/32, so if the instructions are right (which doesn’t fit to my working setting), you could try that.

Edit: I have to revoke to what I’ve written. My server also has /24 in the end.