I almost forgot the use? I would like to upgrade my Desk Lamp
Desk lamps don't seem like they need a supply of 18 V to 21 V at 3.5 A and a second supply of 5 V at 1 A.
You are causing a huge amount of confusion by not providing clear information.
In order to receive help, you need to post a link to the specific battery you are thinking of using, and then explain what you want to power with this battery and for how long.
The simplest answer anyone can give you right now is that what you want to do isn't going to work. If you can't explain how you think it is going to work, there is no chance of it actually working.