I used the eagle design files of the WiFi Module - ESP8266 on my custom board and printed and assembled one from PCBWay. In the product page it states that no firmware is needed since it is a self contained SOC with integrated TCP/IP protocol stack. This is true only in the case that I acquire the board from your site or it should be ready to be used in any case. I have trouble accessing the ESP8266 and I thought that there is no firmware on the board.
Hi, thanks for writing in.
It depends on where you get the ESP8266 devices. Certain supplier will put their own firmware on them or none at all, depending. I cannot guarantee that any firmware is uploaded to any device without testing. If it is a product SparkFun sells and states has firmware it is because it goes through a quality control process that uploads firmware and/or tests that firmware is present on the chip.
Hope this helps.
Can I upload a firmware on a ESP8266 that is on a custom board without the need of rewiring, since this is not an option?
The current wiring is that the ESP8266 is connected to the SAMD21 through the Serial1 (i.e. only through TX and RX).
I also have on my device the debug pins that I can plug in the j-link programmer. Can this help?
You might try asking on the [ESP8266 Community Forum, they should be able to help you figure this out. 
Also, the link below might point you in the right direction if you’re wanting to run the ESP8266 AT firmware.
https://www.allaboutcircuits.com/projec … fi-module/](https://www.esp8266.com/)