Establishing connection for OPC UA through private WAN

I am trying to send data through OPC UA using a gateway eWon FLEXY102 with a special 4G sim that enables me to tap into a private network.

The OPC UA client is on the remote server and the OPC UA server is on the gateway(which doesnt have any ping function towards the server, it's pretty much like a mobile device). Right now I can ping from the remote server to the gateway, but when I use my OPC UA client i can't establish a connection using the IP endpoints.

However, when I connect the gateway to my laptop (ethernet cable), i can establish connection to the OPC UA client. Which should mean that LAN connection is alright.

Everything on the gateway is configured to enable opc ua and the default port 4840 is open and let through the firewall.

I should add - there is a NAT at the firewall that changes the IP in the sim to the IP range within the private network and vice versa.

I'm wondering if I have to route LAN connections to WAN ? Or is there something I'm missing ? Newbie here so any help is much appreciated, sorry if it seems that I am missing some info which I'd be glad to supply !!