Long time passed playing with my OAK's before posting here those days, not easy in fact...
Upgrade to latest firmware was successful at office with 2 units, failed with one that I cannot recover. I still need to try serial...
On the 2 successful units, I was able to upload my sketch. Back to home, nothing worked anymore, upload was always failed but oak was connected to wi-fi and particle. Then, regarding the number of messages related to wifi quality, I switched from my classical home network (Same as office, Ubiquity Access Points) to one old Linksys that I put in 11b mode only with a max TX rate of 2 Mbps. Then suddenly all was back again and working !
Thanks for that to the boot with Pin1 connected to ground to force OAK to enter SoftAP mode.
Now I have to understand why I read from port P6 and P5 does not work the same manner... :-(
In conclusion, try to one old and very crappy WIFI, set it to the lowest possible speed to play with your oak, at least for me this is the working solution I found.