Custom DERP troubleshooting

Hi all, I have installed custom DERP on my Pi, leave it at my office for testing, and opening TCP 443 and UDP 3478 to it to test. The installation run smoothly, and the server now appear in the list with its own latency when I run tailscale netcheck.

Problem is it when my client connected (from iOS), I can’t access nor ping anything from there. After a while, the status in Tailscale status changed from connected to my derp to “idle”, even if I reconnected.

I then try to install derpprobe and hosting the config from Tailscale web console in json repo site to try troubleshoot it, but now the copied config doesn’t pass as valid json.

Anyone got a pointer on where I need to go from here, I greatly appreciate it.