Apple Watch no longer displays temperature

The temperature shown does come from the Weather App. Small bits of data on the watch face are known as complications.

Tapping the temperature on the watch face will open the Weather app thus revealing the source of the data. The watch gets the data from your iPhone which, in turn, uses location and data from the Internet. You may find more useful information in the Apple Watch User Guide.

Are you seeing the "Allow your Apple Watch to unlock your Mac" checkbox?

You may need to enable Continuity. Make sure both devices are on the same WiFi network, and Bluetooth is enabled. The article also had an error, it says

"[...] make sure that Disable Automatic Login is unchecked. If the option is grayed out, you might need to click the padlock icon at the bottom of the window to make the necessary changes."

I think they meant that the box is checked.

EDIT: Here's the Apple article you want to reference: https://support.apple.com/en-ca/HT206995