Check if you enable data quota under Settings >> Data usage >> Mobile data

You can switch it on, however I noticed that battery consumed a lot more than usual.

But that Set mobile data limit, you can just untick it.
Else when you reach the quota, the phone will just stop you from using any apps which requires data.

As mentioned by XeactorZ, you wont kena extra charge if you use over the limit, just slow.

It will use Wi-Fi as the first priority and Mobile data as second.

ya, you can add in by click apps > widget > and look for the weather widget to drag it back