I used the phone number and spoke with a human (Amanda on 12/9/2020 if that matters). They know the website is not functioning for KBV.
The automated payment-by-phone system is also not working, and they know it. I had my invoice in front of me this morning, and I entered the info they asked for 5 separate times, rejected as "Invalid" all 5 times. According to the pleasant woman I spoke to when I called back and chose the talk-to-a-human option, when KBV "returned to us (Wyndham)" last year, the database didn't repopulate correctly.
FWIW, the phone # for Wyndham Kauai Owner Services is 866-523-8477. Choose option # 2 for finances, but at the next step, don't bother with automated - there's a further option for a human.