I would really recommend taking that service off-site. We used to use watchdog, however sometimes the service would crash and we would get false alerts making havoc at the offices, or no alerts at all.
Theres plenty of uptime detector out there.
Sounds like plesk did not update the dns zone for your websites.
Id suggest checking that the DNS is actually pointing to your server with every step. It has to reflect in Plesk, aswell as your dns registrar if youre hosting the dns off-site.