On Android, I can set my APN to be either IPv4, IPv6, or IPv4/IPv6.
I'd imagine the same option is available on iPhones. I just don't know where.
I did some further research, and it's an Apple limitation.
They apparently won't let you connect to anything besides IPv6.
So, you'll have to take this up with Apple.
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I also have a Verizon iPhone and it connects to VPN just fine over Verizon. In fact, when I connect my iPad to my iPhone via hotspot, the iPad can also VPN. As soon as the iPad is back on T-Mobile LTE, it can no longer VPN. It's a myth that this is an Apple limitation.
While researching issues connecting to a PPTP VPN from my phone through the TMO network, I came across this post. While I am running Android, I figured the solution I found may help with IOS. Basically, the issue seems to be that the default APN is IPv6 on TMO while the VPN I am connecting to is IPv4. I found instructions for setting up an IPv4 APN on my phone after which the VPN connects. The instruction link is here: https://www.myopenrouter.com/article/vpn-connections-not-working-t-mobile-heres-how-fix . I believe that the MMSC URL in the instructions got fat-fingered at the end; I entered .../wapenc (which is default APN URL on the existing APN) at the end instead of "wapeinc". Otherwise, I followed the instructions as-is and it worked for me.