What customer ID should be used when trying to set up connection to an external bank account?

It depends on your bank and its connection method to QuickBooks Online (QBO), @leelectricllc. Let me discuss this further.

Connecting your bank account to QBO requires entering your bank credentials. It includes your bank username and password you use on your bank's website. However, the process may vary depending on the financial institution and its connection method to QBO.

I suggest reaching out to your bank since they are the ones who can assist and verify your customer ID. Once done, you can enter the correct details in QBO to connect your bank account successfully. Refer to this article for more information: Connect bank and credit card accounts to QuickBooks Online.

You can also upload your bank transactions manually to QBO. 

Once connected, it's important to categorize your transactions to put them into the correct accounts. Then, I suggest reconciling your bank account monthly to ensure it matches your real-life bank statement. 

Let me know if you have more questions regarding your bank connection in QuickBooks Online, @leelectricllc. Just leave a reply below, and I'll get back to you.