That's surreal.
In my experience, when an ISP blocks a site, they don't do something stupid like giving users a "Site Cannot Be Reached" message which conveys no useful information. They tell you "this site has been blocked because [explanation]" so that you
know what's actually going on, don't waste your time retrying it (as you have had to do), and can ask them about it if need be.
In the UK for example, some sites are blocked by court order due to piracy.
Whatever the problem, no ISP should block any site without being able to give you specific and concrete reasons for it.
I'd find a real ISP if I were you. If they are blocking RD they are likely to be blocking other sites too.
They may even be breaching their contract with you, depending on quite how it's written. Also, I'm certainly not a lawyer but I doubt that any court would deem it reasonable to block sites without giving the users a reasonable explanation,
regardless of what the contract says.
"Risky" is beyond lame!
Do you have that from them in writing?
For the avoidance of doubt, I have no idea at all why they would consider this site risky. Perhaps
@Dave Stephenson has come across this type of nonsense before...