If you’re not too technical, use my NetGuard import file (link below) to auto-configure the app for you. I had to troubleshoot some google system apps to make it run seamlessly. The app works basically by stopping anything from using your data plan unless you say so. You can set apps up to not use data unless you are actually using them as well (screen is on and actively interacting with it). It’s pretty slick. We use about 200MB a month just by using this app. We still get maps/GPS, email, etc. Things we try to avoid doing while on data is using a web browser and video. That doesn’t mean we don’t watch videos. We just indicate that the video is downloaded for offline viewing only while on Wifi (you can do this with Netflix and Plex).
NetGuard link: https://www.netguard.me/
Link to import file: netguard_20181128 – Download to a desktop/laptop and unzip this file with password “1234” (due to wordpress security it doesn’t allow .xml files so using password zip to get around this). Then transfer this file to your phone (e.g. maybe by emailing or certainly via USB transfer).