I had this issue and it only got resolved when I changed the upstream route. I have a post here
What you could do is set up a tunnel to some other router so that traffic going to 207.171.3.17 gets re-routed via some other ISP.
If that works then at least you know if that other ISP has no problems sending NTP replies.
As a test I offer my route if you want to try it out (now that it’s working)