Make sure you have Local turned on so the keyboard will trigger the sounds. With local turned off the keyboard won’t work.
How do you leave LOCAL activated?
Did you change it’s internal Lithium battery? If so… you need to factory reset the DX-7. I had the same problem as you in the past. Also specifically with the DX-7 making no sound. Put in a new Lithium battery and before turning it on there were certain buttons I needed to hold down to factory reset it and get the sound back from the keys. I did this over 30 years ago. I don’t remember which buttons to hold while powering on for factory reset. I’m sure you can find it on Google by asking: “How do I factory reset a Yamaha DX-7 Keyboard”? It won’t factory reset until you hold down a certain button or buttons while powering on. Hope this helps
Electronic Musician magazine May 1989:
“DX7: When you press buttons 16 and 32 simultaneously and then press the function button, the unit will ask you if you want to proceed with the test mode. Pressing Yes will begin the tests. To proceed with each test, the Yes button will increment to the next test, No will decrement. Note that some tests require a RAM cartridge or the performance pedals, the Yes button can skip those tests for which you may not have the necessary hardware. Voice data should not be affected.”
Hope this helps,