If speedtest to the closest server to you gives good latency and speed results then it is not "your" connection causing the problem.
It is highly unlikely both you and the server buy services from the same ISP. If they did you might have a chance since traffic generally does not leave a ISP network unless there is no other option. If it was in your ISP network you might eventually find some engineer that you could harass long enough to fix it. In most cases though the problem is between ISP so you have no hope.
There really is no solution to this problem. Something that might work but will take a lot of research and testing is to use a VPN service. It should make no difference, in fact it should always be slower because of...