Setting up remote access to any network that is operating on a mobile LTE (3G/4G) network is never easy, even for experienced IT engineers. Unfortunately, Digital Yacht just do not have the resources or depth of knowledge to support our customers in doing this on an individual basis.
When your remote network is “hidden” inside a mobile network that abstracts all external connections to the internet, the only way to create a VPN is for the 4GConnect/4GXtream to initiate a connection to your VPN server or shared cloud server that does the data routing for you.
Our 4GConnect is based on OpenWRT, which supports two different VPN services; OpenVPN and ZeroTier, plus a couple of tunnelling protocols that could be used as an alternative to VPN. If you do not have the services of an experienced IT engineer to help with setting up a solution, then I would look for examples of connecting to a remote OpenWRT network over LTE using VPN services.
Here is a link to the OpenVPN documentation on the OpenWRT site and it covers using either a Client or Server setup. Hopefully with this information you will be able to find the relevant settings in our 4G products to setup your VPN.
https://openwrt.org/docs/guide-user/services/vpn/openvpn/start