I am testing the Nova modem and found the following:
1 - Using the “high gain” stick on the antenna at my house location signal strength at 99 all the time and at my office in my lab cell service is sketchy. I get 4 to 6 but it does connect and send.
2 - Using an external antenna like the Adafruit Right angle Mini GSM/Cellular Quad-Band Antenna 2dBi SMA Plug and an adaptor cable. At at my house i now get 7 to 8 and at my lab I get 11 to 16.
Conclusion the external cellular antenna works really well.
RE: charges - If you want to use SFTP to transfer files at many intervals per day over a month no matter what service you use its expensive. I used nethogs running on a pi to evaluate how much data is transferred and a 1.5 k data file costs about 8.5k of overhead.
I evaluated MQTT using mosquitto on the pi and cloudmqtt as the broker. I created a python script to connect (qos1 overhead about 500 bytes). When publishing messages the overhead is about 180 bytes. I decided to send my data packets (500 bytes) data every 8 min which I estimate to cost about 3.00 per month with the 1.00 base cost.
I am preparing to send a few hours worth of data to confirm the calculations.
I hope this helps.