Bank Transactions not imported for 3 months, then resumed. Would like to add them to register manually and have questions in this regard.

Welcome to the Community, @Lj0552.

Let me help you add past transactions manually in QuickBooks Online.

Before doing so, I would like to know when did you connect your bank account? When you connect a bank account to QuickBooks Online, some banks will allow QuickBooks to download as far as 90 days before. 

Also, make sure to know the date of the oldest existing transaction currently in the account to avoid duplicates.

Here's how to check:

1. Go to Settings or the Gear icon ⚙. Then, select Chart of accounts.
2. Find the account you want to upload the transactions into. Then, select View register.
3. Find the oldest transaction, then write down the date.
   Note: Usually, the oldest transaction is the opening balance.

To enter past transactions to QuickBooks Online, manually upload them to QuickBooks Online. Follow the steps below:

1. Go to Transactions, then select Bank transactions.
2. Choose the blue tile for the account you want to upload the transactions into.
3. Select the Link account▼ dropdown, then select Upload from file.
   Note: If your account is not connected yet to online banking, select Upload from file.
4. Click Drag and drop or select files and select the file you downloaded from your bank. Then, select Continue.
5. In the QuickBooks account▼ dropdown, select the account you want to upload the transactions into. Then, select Continue.
   Note: If you're new to QuickBooks, you may not have an account to upload the transactions into. If you don't see one, select Add New ▼ from the dropdown to create a new bank account.
6. Follow the onscreen steps to match the columns on the file with the correct fields in QuickBooks. Then select Continue.
7. Select the transactions you like to import. Then select Continue.
8. Click Yes.
9. Hit Done.

Here is an article for the detailed steps: Manually upload transactions into QuickBooks Online.

For future reference, you can read this article to help you resolve the issue if your bank is not downloading transactions: What to do if you get a bank error or can't download transactions in QuickBooks Online.

Feel free to comment below if you have questions about manually uploading transactions to QuickBooks Online. I'm always here to help. Have a great day.