No access to certain websites using ipad pro

If you are using a commercial VPN service, in so doing you will very likely be using the VPN service-provider’s DNS servers to resolve links to both direct and embedded content. As such, DNS may be filtering content that you can directly resolve using your browser or Apps. When VPN is disabled, you’ll instead be using your locally configured DNS server settings.

As such, use of a VPN can indeed influence what can - and can’t - be resolved by your browser and Apps. This feature can be constructively used to provide content filtering and Ad-blocking.

Any Vpn or related service running? If yes, you can temporarily turn it off to check.

You can still use the VPN should you have cause to do so, but you perhaps need to understand (as you now do) some of the limitations of doing so - such as the potential effect upon DNS and how it works.

At a technical level, your VPN is forcing all your network traffic - including your DNS requests - through the VPN tunnel. This is known a “no split tunnel” - and from a security standpoint is perhaps appropriate.

DNS services are often configured to filter some content/links - or traffic that may come from potentially malicious websites.

You’re very welcome.