It turns out my new iPad removed PPTP since iOS10 due to outdated and unsecured protocol of PPTP, and there is no 3rd party app for PPTP VPN clients for iOS12.
So the only solution without jailbreak is to upgrading my old VPN gateway (running on Windows Server 2003) from PPTP to L2TP IPSec.
Simply select “Allow custom IPSec policy for L2TP connection” and type in a Pre-shared Key in the RRAS property.
On the VPN client side, all you need to do is to select L2TP and type in the Pre-shared Key under Security tab.