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.