Welcome to the community @Kenneth_Cheng !
Tip for sharing information
Pasting your n8n workflow
Ensure to copy your n8n workflow and paste it in the code block, that is in between the pairs of triple backticks, which also could be achieved by clicking </> (preformatted text) in the editor and pasting in your workflow.
```
<your workflow>
```
That implies to any JSON output you would like to share with us.
Make sure that you have removed any sensitive information from your workflow and include dummy or pinned data with it!
Your issues sounds somewhat similar to the one discussed in Inconsistent HTTP Request Node Connectivity to Serveo.net URL in n8n. There it turned out to be DNS related too.
What is the actual domain in question?