Port forwarding on a mobile carrier which doesn't allow port forwarding

Hey everyone,

So I have a remote server sitting in my office which uses a USB 4G modem for internet connectivity, I would like to send traffic to this machine to port 3000 (I have a proxy server listening on this port).

The problem is that the mobile ISP doesn't allow me to port forward and I need to find another solution. I have tried https://remote.it/ which works but I would like to use my own solution and not rely on them, how are they doing this and how can I replicate that myself?

How can I "forward" ports when it's not supported by the ISP? Are they using some kind of VPN solution for that?

Thanks.