In beer/cider making, when fermentation is done you rack it to a secondary bottle for carbonation but that takes more time and equipment. I go the easy route and force carbonate the cider. I built myself a carbonator similar to this one:https://www.instructables.com/id/DIY-Soda-Water-%26-Home-Carbonation---Pays-For-Itsel/
You've got three weeks until your cider is done, this should be your in between project. They are very handy, you can make your own sodas, you can re-carbonate flat soda, they really do pay for themselves.
I bought a 20lb CO2
tank at a swap meet for $10 and it had a regulator. I traded that tank in for a new full tank for another $30. I bought a cheap air compressor kit
from Harbor Freight
Another thing you will need is a carbonation cap that fits a 2 liter bottle.
Here is a good video of how to make a carbonation cap.