I believe the problem has to do with your ISP and the way that they run things. I think you will find that if you use a VPN to override your ISP's blocks that they have in place, your problem should go away.
I have been able to confirm this with a few other cases of this with other users who are using the same ISP that you're on. Simply put, it would seem that your ISP either does not cooperate with our tracker, or that your ISP has some sort of block in place that is preventing you from reaching the tracker. Although I do not know the exact technical cause, it would appear there is either a routing block or a routing problem between your ISP and our systems.
I would be inclined to recommend the use of a VPN not just for privacy reasons, but also to ensure that you are not caught behind routing blocks that I am certain your ISP puts into place for whatever the reason.
EDIT: Come to think of it, I recall trying to troubleshoot the very same issue with a couple of different people who use the same ISP as you do and determined that there seems to be an intermittent routing issue to certain IP subnets (one of which our tracker falls into). As I recall, filing a trouble ticket with your ISP eventually solved the problem, but it was a long, complex, and drawn out process, and the problem reappeared again within a week or two. So… For this reason, I would suggest a VPN.