It is probably your network group administrator policy/server that restricts access unreasonably.

I didn't change any router or network settings, unless windows or dell updates affected?

This problem only happened for nearly a month, I just don’t want to reinstall win11 pro for this, maybe there are other reasons?

If you are on unrestricted network then the denial of access to Dell site is due to a bug on your pc. Try a clean Windows 10 install.

Try download a different browser just for accessing Dell site and hope that goes through. 

I have already tried Chrome and Edge and it is the same, but there is no problem with the mobile phone through WIFI.

I don't think it's browser issue as Chrome and Edge both have the same issue.  The problem could be proxy server, cloudflare, or vpn if you use one.  I think that the Access Denied came from Dell server when your network server could not respond to the request for authentication.  Verify if the issue is isolate to your machine only or it affects all machines in your network.

No, the dell website in other regions has the same problem not just HK.

Yesterday a dell technician advised me to reset winsock, but it still didn't work. The strange thing is that I can access the dell website normally as an administrator.

You can access Dell site all right as admin on this pc but not as a regular user.  This means admin has set up a policy to consider this site “unsafe”.  My company IT admin has a screening AI software that does similar and strange blocking. On some days it blocks Facebook because that is considered a social media thus forbidden, but on other days gives green light.  On rare occasion it even blocks google.  It is a bug in AI.  Your admin may be using some AI in site address screening that has a bug too.