Have reached out urgently multiple times via support email. Support were slow to respond initially and sent me a troubleshooting guide, which I proceeded with to NO positive resolution. I then sent multiple follow up asking for support. Thee last email simply said we've checked and your calendars seem to be working. I went back and clarified again with multiple screenshots showing the issue.

9 days have passed with multiple escalations including contacting AppSumo CTO Chad Boyda on linkedin and no response.

Now going to look at switching out. This is not an organisation you can rely on for sales critical software for scheduling client meetings etc

UPDATE:
Forget TidyCal - 2 weeks worth of back and forth on email with their useless support. Just switched over to another AppSumo lifetime deal (I had to remove the name and repost this but you can find it by doing a quick search) - i'm in love with the new one. Day and night apart. WHY DID I EVER GO TIDYCAL....