I strongly suspect that you have downloaded and installed a "nefarious" app on your phone - usually from a rogue app available in either Google's Play Store, or Samsung Store. Try installing Malwarebytes from the Play Store, run it, and let it try to locate and quarantine whichever app is "guilty" of injecting adware onto your phone without your knowledge. Just because an app is available in the Play Store, doesn't necessarily mean that it is safe. If the "rogue" app is something that you truly need, try to find an alternative to it, from a brand you are already aware of. An example of this would be "SUPER-DUPER PDF READER" (a "dodgy" app), and instead install the likes of ADOBE READER - Adobe being a reputable vendor.