Hey @Pim!
This is unfortunately a general WooCommerce question, and not something related to our add-on for it or to The Events Calendar itself; so I unfortunately cannot guarantee I’ll be all that helpful here 🙁 Your best option for finding a solution to this is to contact WooCommerce support directly, or search online for WooCommerce-specific solutions here.
With all that being said, however, even though you mention having no success with functions.php tweaks thus far, I found this snippet on the official WooCommerce documentation that seems like it’ll do the trick for you: https://docs.woothemes.com/document/set-a-custom-add-to-cart-url-to-redirect-to/
Check that out, I hope it helps! 🙂
— George