ISPs provide their own domain name services (DNSs) - the look-up tables that map human-readable URLs to IP addresses. This means that they can choose not to include some entries. While this might be marketed as protection against malicious sites, it is also the case that privacy and security websites can be blocked, including VPN providers.
Some ISPs allow turning off their protection; see https://libertyshield.kayako.com/article/56-how-do-i-stop-or-remove-isp-blocking for examples. Some sites appear to remain blocked, however, after the ISPs content control is turned off.
In this case, it's also possible though to use another, public DNS instead, such as Google DNS.
On macOS, the DNS can be changed in System Preferences; see: https://support.apple.com/en-gb/guide/mac-help/mh14127/mac. For Google DNS, the DNS server IP addresses are 184.108.40.206 and 220.127.116.11; either address can be used as a primary or a secondary DNS server. (See https://developers.google.com/speed/public-dns/docs/using.)