VPN through 4G/LTE double NAT

[Mitscha]

Hi, my ISP is a cell phone company, and I connect via 4G/LTE router. The ISP uses double NAT, which means that I can't connect to my NAS from the internet. Can I use a vpn-server on my NAS to be able to connect to it from the net, and, if yes, how? I hope it's the right place to ask this, as I've found no special topic about this here. I use both a Qnap TS-269L, original fw, and a Lacie 2big 2 with your firmware. Thank you in advance