So I had the same problem. Batteries new and the voice control worked through the app.
I was ready to buy a new remote where at anger I shouted at it and worked 😅. I realised might be fixable as microfon still works 💪. So after i opened it I found a lot of dust covering the little pinhole and after a good alcohol clean all good works like new.
So before do all the hassle to reset your tv and loose all your data 🙄 (thanks lg support) instal thinkQ app and add your tv ( if you haven't done it already) and check voice recognition.
Hope I could help