leopena_94:
is it the SPI use of both peripherals(ethernet shield w5100 and rfid MFRC522) the problem?
I don't know anything about RFIDs. I am assuming that the W5100 and the RFID are both SPI devices, and so - no. The SPI bus is a bus, this means that a swag of devices can ride on it. All you have to do is have each device use a unique pin to select it. In the case of the W5100, I believe the SS pin 10 select command is in the library, but I haven't I haven't used IOT for ages, and have forgotten most of it. Ironically, my main use of the shield is currently for the SD, hence my bias.
As I said, your talk about SDA pins, i.e. I2C bus, suggests your immediate problem is that you don't know the difference between I2C and SPI. If your RFID really has a SDA pin, you need to change buses, and get on the I2C.
I understand you have had the RFID working, but you need to clarify that, since you can hardly have it working by using the wrong bus.