iPhone 8 Poor Cellular Connection / Inconsistent Reception MI

Apple iPhone 8. Newest software updates. 

Since switching to an iPhone, my mother and I seem to have an issue talking to people on the  iPhone while at / within a mile radious of my home. 

Data / Signal Strength: LTE + 2-3 bars 

Wi-Fi: My broadband cable provider is superb. Router nor modem has changed recently, the technology works fluently at 50-60 mbps on all devices in our home... even a speed test on our iPhones will show 50-60 mbps; just like the laptop and desktop are getting. 

Android Samsung Note 5 didn’t have this issue. Calls placed on Wi-Fi or Cellular Data were generally consistent, without noticeable disruption. 

I have tried disabling WiFi calling, then making calls... still inconsistent/drops. I have tried turning off WiFi entirely, and it’s about the same poor quality. I can hear the person fine, but they can’t hear me. Eventually call “Fails”; as iPhone tells me. 

We had to get switched to the 6GB Mobile Share plan. I have always used roughly this amount, myself. My data speeds become throttled down to 13 kbps. Literally, I have no ideal how the speedtest even finalizing at speeds that slow. The throttling doesn’t usually take place until the last few days of the month, however, the text states “slowed to 128 kbps” so what’s this 8-15 kbps?

I can’t make calls / get consistent internet connectivity on this iPhone on WiFi either... Does Apple have IP data - or - speed throttling?

Since I didn’t have these issues with the Android, and I used 3 GB on average less than I do now, and my WiFi connectivity was so much smoother, I am hoping there is an explanation...