I am able to VPN into my Mikrotik from a laptop using a verizon hotspot, but not my iphone with AT&T — AT&T says is my VPN, but I know thats not the case, is there any setting in mikrotik that might help, or a way to prove the ATT that its their issue?
From what I can gather online it looks like AT&T does block at least some types of VPN traffic. You could try another type (SSTP, L2TP, etc) and see if that gets around their blocking.