Assuming Mango WAN port is connected to the main router LAN port, the main router LAN IP 192.168.1.1 and Mango LAN IP 192.168.8.1 should be on different IP subnets. This is actually the correct setup and you should not change the Mango to be the same as the main router, or routing will not work. The Mango WAN IP should be on 192.168.1.x subnet of the main router.
When you start up the Mango OpenVPN server, turn on Allow Remote Access LAN
With this setup and with OpenVPN connected, you should then be able to open the Mango admin portal at 192.168.8.1 and the main router admin portal at 192.168.1.1. If not, then please post the System Log from the Mango router. I ran a quick test of the setup on my routers and it was successful.
On firmware 4.2.1, the process is as described above and very simple and automatic. Great work by GL.iNet.
Works fine with DDNS by GL.iNet via MENU>Applications>Dynamic DNS>Enable DDNS>ON
Menu>VPN>OpenVPN Server has nothing to fix - leave as is.
Menu>VPN>VPN Dashboard>VPN Server>OpenVPN>Options is where the “Allow Remote Access LAN” switch is.
Wireguard is apparently preferred for about 4x performance. Setup was extremely simple.
Menu>VPN>Wireguard Server - configure and turn on. >Profile add a client. Configure client by download or QR code.
Menu>VPN>VPN Dashboard>VPN Server>WireGuard>Options is where the “Allow Remote Access LAN” switch is.
Download the WireGuard app on the remote device and configure it by file or QR code.