I don't have a solution for this working off the LG Leon which has the identical problem described, but I have a patch that works for me in a related issue that landed me here.
The problem: You can change the data accounting usage cycle for mobile data but can't change it for Wifi. This is frustrating because once you need to change the mobile data usage accounting cycle, Wifi is out of phase.
Most don't care about Wifi usage, but some people have a carrier using certain prepaid rules that gives e.g. a free GB or two of selected social media programs like Facebook, WhatsApp and Twitter but charge for all other usage from the account balance per mobile data MB used, and the prepaid deals you can purchase can be for just around 300 MB. The way we folks make our low cost plans with this benefit is connect to Wifi and these social media apps may at first appear in the wifi tab where you see individual App usage. It is easy to get confused then when you are trying to figure out how many MB the social media apps have used, so you ca subtract that from your all other mobile data usage - and confused I got quickly.
The solution for this: When you see the app in the Wifi tab with the cycle that you don't want, a light bulb went off ... you really jut want to know the mobile use of that particular social media app to subtract it from the total for the mobile cycle you changed with your new prepaid payment. Believe it or not, you can just click on the mobile tab whe you are looking at the app under the wifi tab. Then you will see the data usage for that particular app under the mobile data cycle you set and this is the number you want.
For example, If WhatsApp is basically free, I found it under the Wifi tab only. So I selected WhatsApp and then saw my usage for Wifi WhatsApp, in the wrong cycle dates. Well that doesn't matter. I realized you can then just click on the Mobile data tab and there you see the WhatsApp usage for the cycle time you set. That is the amount straight you subtract from your total Mobile Data use you see in the generic data usage tab and Wifi cycles are of no consequence. This should have been obvious to me but it took a while to realize the phone gave me the info I needed by separating the mobile data use of the App in the mobile data cycle. The confusion was simply because WhatsApp for whatever reason appeared only accessable under the Wifi usage tab and I got confused. The trick was simply to observe on the wifi usage tab you can click the Mobile data tab and then see the mobile usage which did not appear when you tried to get it through the mobile tab originally. It probably has something to do with when I first set up WhatsApp using Wifi. Sorry to present a solution here that is specific, but the topic of this thread is the only place I landed which was close to my problem and it took reading the whoole thread to realize I was asking the wrong question, and all I needed to do was see the mobile data use for the application that by appearing under Wifi made me complicate everything. Millions of people have this issue in certain countries, and most just put themselves at the disposition of the carrier's accounting, but it turn out to be very simple.
Good luck to all.
Good luck to all.