Thanks for reaching out, and welcome to the Community! We strongly recommend power cycling your modem by unplugging it from the power supply, and leaving it unplugged for at least a minute before reconnecting. This has resolved known treasurydirect.gov issues in the past. Please let us know if this resolves the issue!
I tested this and I was able to replicate Mike's issue. Power cycling the modem didn't work. I then used a proxy and the page loaded.
Glad you were able to access the page.
The issue may be a beam issue or an IP issue. It's likely not an OS issue.
I am on a Mac, the link works fine for me.