I can’t update bank transactions from the iPhone app. I have an iPhone XS Max with up to date iOS.

Let's perform some steps to update the transactions, anthony46.

When the device has stored cache, it can cause any bug on the system. That being said, let's refresh its data.

Here's how:

1. Tap the Menu icon.
2. Select Help & Feedback.
3. Tap Refresh Data and then select Refresh.

Afterward, let's try updating your bank transactions again. 

If you want to categorize the transactions, you'll have to log in through a web browser. I've added this article as your guide: Categorise and match online bank transactions in QuickBooks Online.

Let me know how these steps work on your end. Keep safe!