I use Atmel Studio 7 to write RepRapFirmware 3.1.1 bin to the SAME70-XPLD. I did as you did and connect with YAT using the second usb connection on the SAME70-XPLD (not the debug usb). I confirmed the firmware loaded successfully by typing M115 in YAT.
I was able to directly connect a cat5e network cable to the SAME70-XPLD to the laptop network port. From the page: https://duet3d.dozuki.com/Guide/1.)+Getting+Connected+to+your+Duet/7, I followed STEP 8 and typed the command M552 S0 then M552 S1 P192.168.1.15. I set the network interface on the laptop to static and gave the address 192.168.1.10. I was then able to connect to the SAME70-XPLD via the web browser to the 192.168.1.15 address. However, I do get the error that the DUET WEB Control files are missing from the SD card WWW folder. I noticed that the SD CARD on the SAME70-XPLD does not work with the RepRapFirmware. Seems that the SD CARD Detect pin on the Duet 3 6hc is on PA29 but on the SAME70-XPLD is on PC16. So the firmware will need to be changed in order for the SD card reader to work. And of course a bunch of other pins because on the SAME70-XPLD it has that 16MB DRAM that takes up 36 or so pins that the Duet 3 6hc uses. The firmware will need to be changed to reflect that as well. It's a good learning experience though. Thanks.