Hi Miq1,
If you're using an Arduino.cc's Zero then D2/D5 and D3/D4 make TX/RX sercom pairs, whereas on an Arduino.org's M0/M0 Pro it's D2/D3 and D4/D5. This is because on Arduino.cc's Zero, pins D2 and D4 are swapped.
The Adafruit article: Creating a new Serial | Using ATSAMD21 SERCOM for more SPI, I2C and Serial ports | Adafruit Learning System, shows how it's possible to access the sercom ports using the pinPeripheral() function. This function gives the pins sercom functionality without having to modify the variant files.
Assuming that you're using the Arduino.cc's Zero, then D2 and D5 use sercom2, or if you're prepared to forgo the SPI port on sercom4. Pins D3 and D4 use sercom0 or sercom2, however sercom0 is already used by D0 and D1.