We have been contacted several times by Peplink customers who asked us why they are no longer able to setup or use a PPTP connection to our Peplink routers. In almost every case it turned out they had upgraded their device to iOS10 or macOS Sierra.
As of iOS 10 and macOS Sierra, Apple has completely removed PPTP VPN from their built-in VPN client. The main reason for this is that PPTP has many known security issues.
Instead of using a PPTP connection, we advise Apple users to configure a L2TP connection to your Peplink router, as described in this forum post: