Do you mean that you can’t use the webmail interface with that VPN? When you say that Fastmail is “blocking access”, do you mean that you get an error message which states this? Can you view this webpage?
https://www.fastmail.com/about/
I also suggest trying a trace route as described near the end of this Fastmail help page:
https://www.fastmail.help/hc/en-us/a...nection-issues
There might be a problem with routing where Fastmail can’t be reached rather than Fastmail blocking the connection.
Bill