@Tolkien A general problem with powerbanks is that when they see no (or a very small) load they will turn off. Most of them do not work with our IOT gadgets. They are created to charge phones and then turn off.
The only thing I have found that works is this:
I am sorry but I don’t remember where I got the board and you need to do some soldering.
You can charge the battery on on end and power the device from the other.
Or connect the 18650’s directly to the device as @mc-hamster suggested.