on 14-02-2019 06:11
14-02-2019 06:21 - edited 14-02-2019 06:21
You need to start by checking your phones settings to see what is using data. It's possible that your WiFi could be poor and the phone has used mobile data so if you want to guarantee only WiFi use, turn off mobile data on the handset.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone7 (EE)
on 14-02-2019 06:26
@Nathan1 If you aren't using your mobile data, you need to switch it off in your settings. You also need to check to see which apps you have running in the background, and which apps are updating automatically. Many apps, such as Facebook are data hogs. I don't know what phone you're using, but I'm able to set my Android phone to only update over WiFi. As one of your complaints is of poor coverage, make sure your phone is unlocked, and get Pay & Go sims from the other networks to see which one suits your needs best.