Hello Abboud-
There are a couple of ways you can fix this, I’ll go over one easy way, and one that might be a little more effective, but takes some coding on your part.
First, the easy way. In Performance > Page Cache > Advanced, you can use the options Never cache the following pages, Never cache pages associated with these categories, or Never cache pages that use these tags.
That way, if your reservations are always on a specific page, you can just omit that page from the page cache. If you have reservations on more than one post or page, you can either categorize them or tag them to omit them from the cache.
The more advanced way will let you keep the pages cached, just excluding certain fragments from the cache. You can use these instructions to add sections of your page templates that should not be cached and always be loaded dynamically.