If the multi-currency option is switched on but still cross currency settlements are not happening suggests that the bank and supplier set-ups are done before the chnage?

Yes, Dilantha-Kumara. Let me share more details with you.

Cross-currency settlements only work after you turn on the Multi-currency feature. So, if they are not happening on your end, it's likely as the feature was enabled after you recorded transactions that are supposed to be cross-currency settlements. Also, you are right that supplier and bank accounts (in different currencies) need to be set up only after turning on the Multi-currency option. 

To fix this, recreate your suppliers and bank accounts in the correct currencies, re-record the transactions, delete the incorrect records, and inactivate the old accounts. 

I’ll include screenshots to show how to inactivate your bank accounts and suppliers.

For Bank accounts: 

For Suppliers:

You can check out this article for more info: Set up and use Multicurrency in QuickBooks Online.

Please don't hesitate to reach out if you have additional questions about the multicurrency feature.