How to get around Double NAT from the ISP?

Not really. The only thing you could do would be to tunnel through their network to a server somewhere else, and set up routing rules to route everything through the tunnel instead.

For example, if you created an IPSec tunnel (which might not even be possible with the double-NAT, but you might be able to get away with L2TP or, dare I even suggest it, PPTP¹) originating at the site and connecting to something under your control, you could then assign it an IP address using 1:1 NAT or something gross like that.

_{¹Yeah, don't do this. PPTP is really not a VPN, it's just a VN. There's no privacy on PPTP. The main advantage of it is that it's so easy to get running and so incredibly widely supported. But that said, there's a lot of new equipment that does not support PPTP at all because of how easily it is snooped upon. So if you do go down the PPTP route, just remember that it's no more secure than sending your stuff over the internet in plain text.}