My transaction review/categorise feature has disappeared off the mobile app, does anyone know how to get this feature active on the mobile app again?

Thank you for posting here on the Community page, lyledaniel01.

I'll share steps so you're able to review or categorise your transactions back into your QuickBooks Mobile App. 

The disappearing feature might be caused by a cache-related problem. By clearing its cache, you can remove that historical data and access QuickBooks with a clean slate.

Mobile App:

1. In your QuickBooks Online Mobile App, select the Gear icon. 
2. Choose the Refresh Data menu. 
3. Tap Yes.

Android Settings:

1. Go to Settings and select Apps.
2. Choose QuickBooks Online and click on Storage.
3. Select Clear Data.

After dealing with your transactions, you can start your reconciliation routine: Reconcile an account in QuickBooks Online. 

Don't hesitate to post here in the Community if you have concerns about categorising your bank transactions in QuickBooks. I'm here to help. Stay safe.