I just read through the current online Pages user guide for iPadOS and the Books version too. Not one word about entering or editing formulas, only mathematical equations. Also Googled this issue and found nothing. That makes Pages tables useless in iPadOS v18/18.0.1.
If I create a new Pages v14.2 document on Sequoia v15.0.1, enter a table and a table formula, and then save that to iCloud Drive, Pages v14.2 on iPadOS 18.0.1 honors the equation, but as far as I can tell, gives no user access to the formula editor.
I just submitted a bug report to the Pages v14.2 for iPadOS product team about this issue.