There are some known API issues with the 4.0.0 Beta3 firmware for Slate Plus that cause it to render improperly on the App.
Please upgrade to version 4.0.0 Release1.
https://dl.gl-inet.com/?model=a1300&type=beta

Are you configuring the wireguard on the web?
The App don‘t support the use of VPN profiles stored on the firmware, this feature is scheduled to be added in new version that release in October.