I do appreciate the steps you've performed to get the Advanced Search working, Alert SIx Ltd.
Aside from clearing the app's cache (Help > Reset App Data), we can try uninstalling and reinstalling the app. Then, use the Advanced Search option to look for old transactions. You can use this link to download the app for QuickBooks Online.
Meantime, you can use one of our supported browsers to manage your transactions. For more details you can check out these articles:
- Customize invoices, estimates, and sales receipts in QuickBooks Online
- Run reports in QuickBooks Online
I'm just here if you need anything else. Take care and wishing you all the best!