A couple of suggestions that may be from out in left field:
Go to settings>Safari. If your search engine is Google, consider DDG. Block pops are on. Prevent Cross site tracking is On.
Click Advanced>Website Data. This will tell you what websites have data held for Safari.
Now go back a page and click Clear browsing History. This can include website data clearing.
Lastly, go back to main Settings, click on Privacy>App Privacy Report>Turn on Report. In a week of usage, go back to this page to see what Apps are accessing the network as well as web pages in Safari that are hitting the network. This, if nothing else, will give you an idea of who is most often calling back to the mothership (expect to see Facebook even if you don’t really use it).