Thanks for getting back to this thread, @gijeet.

Allow me to add more details with regards to your banking concerns in QuickBooks Online (QBO).

Connecting your bank account to QuickBooks is optional. There's no need for you to connect with your Financial Institution if your customers won't pay you online or if you don't need to download your transactions from your bank. 

Since your customers pay you by mailing a check, you can simply create an invoice or sales receipt to record them in the program. 

If you're using a journal entry to record these transactions, you can still reconcile them without necessarily connecting your bank to QBO. 

All you need to do is to download your bank statement and compare it with the ones you've entered in QuickBooks.

For more info about this process, please check this article:  How to reconcile your accounts QBO.

Keep me posted if you need anything else or if you have other banking-related concerns in QuickBooks. I'll be here to help. Have a wonderful day.