Generally there are only so many places where things can go wrong when a device won't charge, so let's take a look and see if we can narrow it down on your calculator.
The battery is an obvious place to start, and it sounds like you've done your due diligence there, so we'll assume the battery is fine and move on.
Next thing to verify is the charger and cord if it's separate. Does your charger indeed work on that "different device" that you used to check the battery with? Can you measure the voltage coming out of the charger, and if so, is it correct?
What happens when you plug in the charger? Does it give any indication that it's plugged in? Does holding the plug at different angles make any difference? If so, it could be the plug; there's a guide to replacing the charging port for your device and it appears the only thing physically securing the port to the board are four circuit board traces. In other words, that port isn't going to withstand any kind of abuse whatsoever without ripping the traces off the board, so that's definitely something you'll want to check. Pull up the Power Connector Replacement guide and open your calculator up so you can get a good look at the power port.
Texas Instruments TI-Nspire CX Power Connector Replacement - iFixit Repair Guide
If you want, take some pictures of the port itself and we'll see if we can tell if there's any damage or not.
Just a note of caution: Should your calculator have the same problem that's being fixed in that guide, I would not recommend wiring it like he did. TBH, that's an awful repair job; you'll want to keep those wires as short as possible and definitely use insulated wire. We can help out if that's what you end up needing to do.
So check out the things I've mentioned and let us know what you find.