Why Can't I Set My Location in the Weather Widget?

Well, you have to have Wi-Fi enabled for Location Services to work. You need to make sure Ethernet is at the top of Network Preferences. Thus…

If it's not, you can just drag it to the top.

I also have FiOS and I still get 900Mb/s something up and down.

Welcome, ProducerKen, to Apple Support Communities!

Have you not been able to find where you set Location to your current location?

I had to “fiddle around” a bit to find where that was set.

Right-clicking on the Weather Widget brings up a contextual menu: select Edit “Weather”.

The widget will enlarge and “flip over”.

You can then select the Location to be a desired City, or My Location.

Go into System Preferences > Security & Privacy > Privacy Loctaion Services. Untick Weather and then tick it again.

FWIW, you can have Ethernet and Wi-Fi enabled.

When you say nothing happens, are you saying you click on Location a search window doesn't open with a search field to type in the name or zip code?

That's good. I was going to suggest you try a restart but lunch interrupted. ;)