Are you expecting wet cell voltage levels from an AGM battery? Then you don't understand AGM batteries. Did your vehicle originally have an OEM AGM battery? If not, that's your problem. Put a wet cell in there and get on with life.
I worked at a place that replaced all their wet cell batteries with AGM's, real high end quality stuff. Tried every brand. They installed zero, none, nada charge limiters. After 2 years we had 3 pallets full of dead AGM batteries. All warranty refused, overcharging. When I left they were still sitting in the corner.
AGM batteries have very specific temperature and charge requirements. If your vehicle is not made, or modified to honor that, you'll have problems.