Find the culprit and remove it
Install an AV scanner, scan, and remove.
Find the culprit
If the symptoms appeared immediately after installing an app, that app may be the guilty party
If you get ads in the notification bar, then long tap those and then touch on All Categories to learn which app is displaying the ads
Go to Settings -> Battery and monitor usage. If you recently charged, Battery usage data is not available. Wait a few hours for the data to become available.
Go to Settings -> Network & Internet -> Data usage -> Mobile data usage / Wi-Fi data usage
Found? Then remove.
Remove the malware
Go to Settings and tap on Apps & Notifications
Locate the app
Tap on it, and choose Uninstall
If you see the “Uninstall” option is greyed out or not there at all, then it means that the app has gained privileges.
Remove admin rights
Go to Settings -> Security & location -> Device admin apps
Tap on the checkbox and then tap on “Deactivate this device admin app”
Go back to the Apps & notifications to remove it
If removing admin rights did not work, do a Factory reset
If a factory reset did not work, it is probably a very persistent malware (like xHelper) You may wish to analyse it and/or get help.