If they work on different subnets, sound like you have a your interfaces confused. Both APs should work on the same subnet providing they are connected to the same interface on the UTM.
You will have the same issue if you use XG, it is not UTM/XG issue but a user configuration issue. You shouldn't be using the WAN port on your APs but a LAN port if they are on the internal network.
If they work on different subnets, sound like you have a your interfaces confused. Both APs should work on the same subnet providing they are connected to the same interface on the UTM.
You will have the same issue if you use XG, it is not UTM/XG issue but a user configuration issue. You shouldn't be using the WAN port on your APs but a LAN port if they are on the internal network.