At first it did not read card, but I got the version number printed. After I changed some wires (I checked multiple times if the wiring was off, but I think it was right, so I just did it again) it started to work.
I played around with it, and tried different setups. I am a complete beginner, and I think I have supplied the RC522 with 2 18650 batteries. Now after these tries it stopped working.
Could I damage the board with the 2x18650? I tried the above sample code today with no luck. I tried to check for connectivity issues with a multimeter, and it seems all the connections are present.
Now after supplying the code in the site I linked I got no printing in the Serial Monitor. Not even the firmware version. Is it possible that I damaged the RC522 board? Is there any way to check it? Its led is on but apart from that, nothing. The NodeMCU seems to be working, I can get it to connect to WIFI for example with other codes.
Also the SD2 SD3 pins on my NodeMCU ESP 8266 was shorted, they were soldered together, so I eliminated that. Have I made a mistake? Should those pins be soldered together?
@gerdos
Get another RFID board and see if the code works on that one. If it does you have probably fried the board. They are only rated for 3.3vdc (as far as I've seen), so if you are directly powering it off 7.4v, then there isn't much hope for it. Check datasheets before powering any modules off of anything higher than 3.3v. Quite a lot of arduino modules don't even like 5v, so also check before using the 5v pin on a mega or uno (and the others that are 5v). Does the LED on your current RFID reader still light up? You MAY be able to repair it, depending on which component went pop.
that would kill it alright.