Using your phone Wi-fi, and can't do it...

Try turning off your phone data.

I have a Yi Action Camera (and just bought a Yi 4k camera for live streaming, if I can do it) that I had previously connected to the Yi app.

For some reason, the last time I tried, a couple of months ago, I couldn't do it after attempt after attempt. Tried Saturday, same thing. Uninstalled app. Reinstalled app. Reset camera, rebooted to factory settings. Confirmed I had the latest firmware. Over and over and over again.

I pulled manuals from the internet, checked diagrams, checked YouTube, read forums, asked questions. Over and over and over again.

Finally found a 90 second youtube video in a foreign language. You could tell he clicked something on his phone. Founded a broken English 3 step version. Basically all of the steps were turn your phone data off, and try again.

Boom.

Sometimes I hate technology. No, I always hate it. The Luddites had it right.