iPhone 6 and 6s LTE signal problem

Hi, i have an iphone 6 and LTE doesnt seem to be stable. Whenever i acquire LTE it only takes a few mins then it goes to 3G. Mostly LTE shows 2 dots for the signal. I already had my iphone updated to ios 9.1 and carrier is already updated. I never had this before with my iphone 5. I already tried to restore, reset network settings, aiplane mode and reseat the sim but still the same. I also tried with my sister iphone 6s and same problem occur when getting LTE. i already called my carrier and said that LTE is active in my plan and APN is correct. Even when im in other location same problem with LTE.

I was given with a new sim but still same problem. What could be the problem with this?

Just one question though for someone who knows this, if the Carrier doesnt have any VOLTE capability yet and only has LTE for data will this affect the service for LTE when using iphone 6 and up? Because when i check in Cellular Data i have an option wherein it has VOICE & DATA then once tapped you are only allowed to select LTE/3G/2G but theres no option to select LTE only for Data.