We've enountered a lot of Issues with VPN on iOS devices.
A core-point is Apple's DNS-Privacy: When connected to a wifi, apple by default is ignoring the assigned dns servers (and therefore any dns assigned by the tunnel) and instead is using the apple cloud dns, "to protect your privacy" (at least that's the reason they claim)
You can set the dns-server for a particular wifi to manual to resolve this. Just becomes very unhandy, if you have hundrets of clients dealing with that "feature".
Your observation about the IPV6 / IPV4 difference might be the problem:
Your iPhones provider is using IPV6, and your ipv4-connection is then only an ipv4 over ipv6 tunnel (In our country referred to as "Dual-Stack-Lite / DS-Lite").
Here you'll have limits on the usable ports - depending on the provider. He might have decided, that a common user needs to use port 80 and 443, therefore created the proper rules for that, but everything else well not be forwarded.
(Your Phone would be the router here, IPV4 only over nat'd IPV6)
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Your best option here would be to make your vpn-server ipv6 capable , OR (that's what we did): Use Port 143 - that's IMAPS - nobody is using imap nowadays, but that port is most likely served by your isp.