On my Iphone 6S, I have had for 2 years and I have had 4G for that whole time. For about 2 weeks, I have had 3G. How do I fix it ?
Have you moved around or are you saying you are in the exact location as when you use to get get 4G and now receive 3G there?
As William asks do you get 3G everywhere you go? If so it sounds like an issue with the phone.
Check you haven't set it to forced 3G mode.