Unroot phone without root access

I remember rooting my LG G3 like two years ago with Stump Root. Now I don't use root anymore. My bank app doesn't work because my phone is rooted. Now I want to remove root from my phone. I found out that I can remove root access from my phone when I remove the su binaries from the phone. I downloaded ES Explorer to do so. ES Explorer told me that I don't have root access. Now I can't remove the su binaries. How do I unroot my phone now? And why thinks my bank app that I have root even though ES Explorer and root checker both tell me that I don't have root access?