You can use the below steps to connect to VPN using PPTP protocol on Mi Router 3C
Step 1: Open Mi WiFi App on your phone and navigate to Settings -> network settings -> VPN settings.
Step 2: Click new VPN and fill in the details:
Name: Provide any name of your choice
Server: Any server mentioned under https://network.limevpn.com/
Username & password: LimeVPN username and password
Step 3: Click OK to save the VPN connection and enable it.
You should now be connected to the VPN server. Visit https://ipchicken.com to confirm if your IP address has changed.