I heard that wifi only disappear when wan IP address becomes 0.0.0.0. that's how it is designed but I don't really understand why. Maybe you can give them feedback.

I guess you can do a firmware upgrade after reset the router:

the default key is on bottom lable showing wireless password. If the default one doesn't work, you can reset the router and try the default password again

they have the guide: http://www.tp-link.com/us/faq-999.html

emm... sounds strange, during the process to add it again, is there any error message? sometimes the phone will switch automatically from the original tplink smart plug wifi back to your router as...

flashing blue/amber means it is in app config mode, so you need to add it again to the WiFi network via Kasa App