Hi Jacob, today I had the same problem as you, and I finally get a solution. My context is more or less the same as yours, I bought the LED Bulb II (Color) on December 2019 and I set up my bulb correctly then, and everything work fine; but in my case I didn’t change to a new router recently, but I changed the password of my router and since then I got the same problem as you.
So what I did today was change the password of my WiFi net (2.4GHz) again and then follow the normal steps to vinculate the device (add new device, select LED Bulb II, reset the bulb switching on and off five times, select your device) and when you arrive to the window where asks to select the wifi network to make the vinculation, here you have to press the 3 points of your WiFi net and select “forget network”, then you must select again your wifi on the list of available networks and now it must ask for the authentication password, you enter the new password and now it must works, at least that’s what worked for me.
I think the problem is that the yeelight app save automatically the old password and doesn’t asks for authentication when you change it or change your router, so it is impossible to connect because it is not the correct password.
I hope this can solve your problem. Let me know any case! 
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