MacBook Air (M1) internet issues: unstable internet, potentially caused by some connections being blocked

Recently my MacBook Air (M1), running Ventura 13.2, is behaving unreliably when it comes to internet.

Webpages seem to load incredibly slow, if at all. It happens in all browsers.

For instance: speedtest.net does not load the actual test, but gives the following error:

DOWNLOAD TEST ERROR

A socket error occurred during the download test. A firewall could be blocking the connection or the server might be having some issues. Please try again later. 

Alternatively, check out our free Speedtest for Desktop apps for more reliable testing.

When I Apple Wireless Diagnostics I get the following results:

You are connected to a Wi-Fi network that cannot communicate with the internet.

Try restarting your wireless router, and if that doesn’t work contact AppleCare customer support for further assistance.

Things I tried that did not solve the issue:

• Booting in Safe Mode
• Restarting the router
• Disabling the firewall

I am not 100% sure when this issue started to show up precisely and what changed.

The problem does not occur:

• On my other MacBook or iPhone on the same wifi network —> So I do not think it’s the router or ISP.
• When I’m running NordVPN —> might be bypassing some connection filter?

I’ve been struggling with finding a solution, and am very curious is anyone can point me in the right direction.

Many thanks in advance!