ISPs block VPNs by different techniques, With port and IP address blocking being the most common. ISPs can search for block the IP addresses known to belong to a certain VPN company. To disable connection ports that are mostly associated with VPN traffic.
This is WRONG
in the UK.
I have been using home working VPNs, of 4 different types for 20 years, across about 10 or 15 ISPs.
There are different types of technology used to implement VPNs, some use TLS (formerly known as SSL) and appear to be a secure web site. Others use IPsec, some use IPsec encapsulated, and then there are OpenVPN and the really new WireGuard. That is just a few.
If there is a problem with a VPN, it is likely to be down to limitations of home routers. In most cases turn OFF any router supplied VPN services, or any "helpers" for VPN.
20 years of broadband connectivity since 1999 trial - Live BQM