The same thing happened to me years ago. When I complained the response was that “unlimited” meant “we don’t block access to the content we provide you”, not “you can expect to download speeds at maximum throughput”. This was many years ago before net neutrality was a buzzword, and it was common for certain carriers (like cellular) to serve up alternative sites or isp’s prevent you from running quake servers or such. Not sure if that’s the excuse now, but it’s vague for a reason so they can exercise that wordplay.