So is this 3G service no longer work if you have a 4G phone or...does 3G service not work on 4G phones.
No, it's not that. I live in a 3G-only area and data works fine, and when I travel to a 4G area (and have LTE enabled) that works, too.
But I don't know what the problem is.
If you are in a SPOTTY 4g area or on a 3g area. Turn off the 4g radio on the settings menu. Settings > wireless and networks >mobile networks >network mode