Running a 12V 1.6A fan on a battery
There's a few caveats here.

Are you sure the battery is 30Ah? If it's cheap and less than about a kilo (based on lithium's specific energy; Wikipedia gives a pretty wide range of values), chances are that it's not really 30Ah, and that's not even including a case, charge management and output boards, etc.

Do you care if it runs at full power? The 30Ah pack has a higher capacity in Wh, but is apparently limited to ~10W on its output, per channel. Your fan, would, at full load, draw approximately 20W. It may be possible to parallel the power bank's outputs, but this is dependent on how the pack is constructed. Even so, the pack may have a protocol or sorts for charging at the full 2.1A, usually dependent on data line voltages.

Given that you still want to use this battery, the item you're looking for is a boost converter. It will take 5V and "boost" it to 12V, but drawing proportionally more current at the 5V side, plus a bit for losses. The good news is that these are dirt cheap (>$1). eBay and Aliexpress have a wide selection, and for a low power load like a fan, these will suffice.

More data on the 30Ah battery would help us help you make a better choice.