When using Safari, and your Gtmhub trial account has expired, clicking on the Enter payment details button appears to do nothing:
This problem is due to Safari browser default settings, which are configured to block pop-up windows. The Enter payment details button needs to open a pop-up window and redirect you to our billing system, so you can update your payment method.
Follow the instructions from this article on Apple Help: Allow or block pop-ups in Safari on Mac. Alternatively use another browser and ensure it does not block pop-ups either.