Thanks Biketool, The problem is the LTE 5G modem from the ISP gives me a 192.x.x.x IP address but my real external IP is 37.x.x.x but my router cannot see it, so services like DDNS don't function. This double NAT situation appears to the price you pay for high speed LTE based internet. I've had Wimax connections (100mb/s) before and a quick call to get NAT switched off (no need to pay for static IP) solved this issue, but Wimax is not available at this address. But from what I have read with this LTE/5G technology NAT can only be resolved on IPV6 not on IPV4, and then only at a price. Getting a static IP doesn't solve the issue, as it did on cable/fibre based systems.
I think I understand your suggestion about VPN out to a server but it brings a couple of issues, security if using a external server "service", which defeats the object really of a secure chat server. But also speed, if I use my own external server and VPN to that, I have no where with 250mb/s connection to place the server. If you still think your proposal will work I would be happy to try it if you can clarify more.