You should be able to establish an IPsec tunnel through a PPTP connection. I'm assuming that ISP is assigning you a world-routable address and that your VPN gateway is also accessible. If not, there may be some routing issues that are causing the traffic not to return. Alternatively, PPTP is notoriously slow and may be causing timeouts for your IPSec VPN. If that's the case, your may need to extend the timeout interval.
Hope this is helpful,
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