I'm finding it hard to narrow down where my issue is because I made some hardware changes and upgraded from a pfsense appliance for my firewall to opnsense running on proxmox. For the most part it works absolutely fine with just a few teething issues. From a vm on the proxmox host connected to my LAN bridge network I can get 900/120 speeds (not as fast as I actually hoped but good enough).

From my subnet where plex is, on both the host and the container it's running in, I can get similar speeds with speedtest cli. And when I stream plex internally, which crosses opnsense as it goes between subnets and through the firewall, I can stream 4k no problem.

But lately I had a friend remote streaming my content from Plex complain they couldn't stream 1080p when they could before. I verified when I use data on my phone that it buffers pretty badly. 

BUT when I turn on my VPN on my phone, still on data, and stream I can get 1080p direct play perfectly fine. The plex dash board shows a healthy stream and there are no issues.

So I can only deduce that going out my network the connection is ok, but something about my network isn't happy with streaming to particular networks in New Zealand at least? Could this be throttling or another issue?