Upload latency in 1000s only for some users

[Dr Dappadan]

I am experiencing massive up latency (e.g. 7000ms) with specific users in the UK, while my ping is low (~12ms). 

Seeing as I've been chatting with Gemini for a while trying to solve this (but it got me nowhere despite trying many things), I'm posting the summary below see if this is familiar to anyone or knows what the fix might be.

I don't remember having these issues when I was in a FTTP fibre area (unfortunately had to move to a FTCC area!

Many thanks for any help
Dan

My Setup:

• OS: Windows 10/11

• Router 1 (ISP): BT/EE Smart Hub 2 (IP: 192.168.1.254)

• Router 2: GL.iNet GL-AX1800 Flint (IP: 192.168.8.1 / WAN IP: 192.168.1.209)

• Audio: 16-bit PCM / 48kHz (tried lowering to Opus 64kbps with no immediate relief).

What I have tried so far:

1. Port Forwarding: Set a UDP port forward for 12000 on the BT Hub pointing to the GL.iNet (192.168.1.209), and another on the GL.iNet pointing to my PC (192.168.8.117).

2. DMZ: Attempted a DMZ chain (BT Hub DMZ $\rightarrow$ GL.iNet, then GL.iNet DMZ $\rightarrow$ PC).

3. Windows Firewall: Manually added Inbound and Outbound UDP rules for port 12000 for all profiles (Public/Private).

4. SonoBus Config: Checked "Use Specific UDP Port" (12000).

5. UK ISP Specifics: Confirmed "Smart Setup" is disabled on the BT Hub.

6. External Test: A check on CanYouSeeMe.org for port 12000 still returns "Connection Timed Out."

The Problem:

Despite these steps, I cannot get the connection icon to turn solid/green. The latency to certain users is astronomical, suggesting the relay server is severely congested or the "Double NAT" is causing a buffer bloat issue that I can't clear.

Question: Is there a specific way to handle the Smart Hub 2 + GL.iNet combo to allow P2P, or is there a way to force a reset of the relay buffer when it "sticks" at 3000ms?