Galaxy S5 has NEVER Updated and is still on 4.4.2

I've had my Galaxy s5 from November 2014.  I purchased it through bestbuy.com and had it activated at my local AT&T store.  Since that time, I have not been able to receive any software updates. Every time that I search for an update in Settings, it tells me that my phone is up-to-date and that I must wait 24 hours to check again.  I also have had horrendous mobile service and frequently, can't receive or send text or MMS messages; drop service completely in areas with full service; and don't receive phone calls.

The version of my phone is 4.4.2. However, both versions 4.4.4 and 5.0 Lollipop has been released since the purchase of my phone yet, I have never been able to get them.

Is this a device issue or a carrier issue?  I'm guessing that my service issues are due to my phone never updating.

Can anyone help me?