@Nealoroni I'm pretty sure your ISP is using CGN in that case.
CgN (or CGNAT/Carrier grade networking) is what they slap across their customer base in order to share a pool of publicly facing IP addresses within a range of IP's...rather than providing each customer with their own individual public IP. Cost saving measure so they dont need to pay for a bigger reserved IP range when handing them out.
Read through my rants above and my final fix after nearly a week of trying everything under the sun.
I reckon Larian is IP filtering to allow only unique IP connections. They should be using mac address which would likely have avoided this.
