Verizon had not even finished with converting all its towers to 3G from 1xRTT before it started a new program to convert them to 4G. Verizon is going to upgrade the towers in high density urban areas first and in rural areas eventually in a year or two or three.
With a 3G/4G card (modem actually) the device tries to make a 4G connection and when it fails it then tries to make a 3G connection which often succeeds if the tower is close enough and nothing interferes with its line of sight signal transmission and reception (from your device which puts out a lot less power). Problem is that after 10-15 minutes the modem will try to upgrade the connection to 4G and in doing so it kills the 3G connection.
With towers that have 1xRTT and 3G transmitters there is an internal setting that you can make to force the modem to never make a 1xRTT connection which is going to be extremely slow. If you do not do this the modem chooses whichever transmitter signal is stronger and not the one that will provide the fastest speed.
I switched back to a 3G modem only and all my dropped connection problems went away. It was the modem and the tower causing the problem as my signal strength is excellent thanks to the use of a mast mounted parabolic grid antenna and a powered signal amplifier which in tandem quadruple the signal from the tower.
Verizon has the most coverage with regard to data as well as cell phone and there are public maps that show towers in your area and whether they have Sprint or Verizon or AT&T transmitters. The companies lease space on the towers for their transmitters. The nearest Verizon equipped tower is 3 miles from my house and the nearest Sprint equipped tower is the same distance but on the other side of a hill so the only option was Verizon.
Millenicom resells both Sprint and Verizon data services at a lower price and with double the cap on monthly data usage and it is month to month and they provide the modem (card and USB types) at no charge. I have used them for more than two years and they have been great in every respect.