I had this about a month ago, then I just suddenly got the weather widget back, all I do is sync it daily
The phone with the slash through it (in place of the weather app) on my Vivofit 4 meant that my phone was not connected through Bluetooth to my phone, even though I had paired through the Garmin Connect app on my phone. I simply shut off my phone's Bluetooth for a second and turned it back on, revealing the option to then pair my Vivofit 4. I held the button on the Vivofit until the WiFi looking icon came up and then it paired. The watch then showed the weather.