So I see this question pop up all around the web, yet I haven't seen any answers to fully solve this issue. Bear with me this might be long...
I am trying to use the RFID-RC522 modules for an escape room idea. I am using the MC522 Library and the SPI library to read the readers and then switch a relay. Every time I try and connect one reader to the arduino, it works perfectly. Then, when I try to use a second reader, the system doesn't recognize either of the two readers at all. Here's some of the things I have tried to combat this. One thing I discovered was that the MISO pin (when hooked up), was causing the first reader to not be read. So with all this knowledge, I tried the below things:
Use a 3.3v to 5v converter to see if it was a power issue. (Didn't Work)
Use 4.7K resistors on the MISO pin to see if that was causing the issue. (Didn't Work)
Connecting the RFID readers to separate 3.3 power supplies. (Didn't Work)
Use a 3.3v to 5v converter to see if it was a power issue. (Didn't Work)
Does that mean you use the chip directly (no breakout board)? Most board include a level converter. These level converters often are the problem. Post a link to the schematics of your breakout board! You description let me expect a wrong level converter where a CS line going high doesn't put MISO tri-state.
Use 4.7K resistors on the MISO pin to see if that was causing the issue. (Didn't Work)
Bad idea, never works on an SPI bus.
Connecting the RFID readers to separate 3.3 power supplies. (Didn't Work)
Depending on the board that fails because the necessary power is not provided to the chip.
If your vendor doesn't provide schematics of the board (I never buy such junk) you have to reverse engineer it an create the schematics yourself (using a multi-meter [beep] and a magnifying glass this shouldn't be that hard).