When on T-Mobile, can't connect to personal VPN

I run the built-in PPTP VPN off my Asus RT-AC66U. When I'm connected to T-Mobile, the VPN from my 6P won't connect - it just says "Connecting" until 15-20 seconds later it says "Unsuccessful." When I am on Sprint or on Wifi, the same VPN connects successfully.

Before I was on Project Fi, I had to manually adjust my T-Mobile APN to "default,supl,mms" to allow the VPN to connect. Not sure if this is the same issue I experienced then or not, but since Fi hides the APN settings I can't try it, anyway. Anyone have a solution?