This is something I've worked on for years. I'm formerly a P.M. for both trail cameras and mobile DVRs that people want cellular remote view on. I've also built several custom cellular products, mostly for law enforcement.
You come into several different, complex issues.
First: Most of these products come from Asia, where the GSM bands are functionally different than they are in the USA. Second, traditional quad band GSM (which is 3G) in America is on it's way out. (as what happened with the old 800mhz analog bands a few years ago. Ask anyone who bought a mid 90s vehicle with Onstar).
Second: The frequences that we use in the USA for 4G (LTE, 700mhz) are not used any where else on earth, so Asia vendors don't make equipment that uses them. On top of that, with the FCC constantly creating and selling new frequency blocks, it's a nightmare to try to stay on top of those things. Think of how many different technologies have come out in the last 5 years. Clear Wire (remember that? 2100 mhz). LTE on 700mhz, 2400mhz.
On top of all of that, Verizon is very uptight about allowing things on their network, so even if someone did make a 700mhz LTE product, they would still have to get it approved and certified before it would even work on their system.
Reconyx makes a Verizon certified LTE trail camera, but it's very expensive.
'Lil Acorn' makes a GSM trail camera that works on T mobile and ATT GSM networks.... for now. Eventually those GSM networks are going to be shut down. Those GSM cameras functionally will not run on Sprint or Verzion because they are not GSM based.
Verizon using SIM cards for LTE has confused a lot of people. The SIM card that's in your Galaxy S5 right now won't do anything if you stick it into a Chinese GSM trail camera.
This is the same problem with mobile DVRs, most of which are from Asia. Even though they may say that they have SIM cards, they are GSM based on bands that are iffy in the US at best. On top of that, they require some pretty odd ball back end servers to communicate (which you have to build) using some fairly old networking architecture from my experimentations.
Adding a USB port with the presumption that you will stick an aircard into it (which will allow you to upgrade as your carriers upgrade) is a great idea. However, the software devleopment team for that camera will have to stay on top of that constantly, as new aircards are coming out every month. Also, the camera will have to have an actual operating system in it to be able to detect that as a network device.
I worked for many years with a mobile DVR manufacture that did this. Their product had a USB port that would detect cellular aircards from Sprint and Verizon, but it was a constant challenge to add driver support for them as fast as new devices came out. They were not an Asian company, but they were also not based in the USA. So, I was their technical support for understanding the cellular network in America, and that can be a full time job.
On top of all of that, streamimg an IP camera is going to be a bandwidth hog and may max out even the best 4G signal on the best days. The mobile DVR mentioned above had a highly compressed H264 stream that I actually made work over a sat network link... but as you can imagine the video quality wasn't impressive.
Some pole mounted city wide camera systems that have embedded NVRs (I think GeoVision) in them will stream a low res secondary stream over cellular to a head end video wall at police headquarters, then if there is an event, they can go back and remotely extract the high res video after the fact.
The best solution right now is to use an external cellular router like a Sierra Raven X or CradlePoint. Both have hardened versions that are specifically designed for the carriers that you want to use. The down side? Additional current draw for remote applications and additional costs around $600. Also, you better have unlimited data on your rate plan because you're going to destroy your data cap in a matter of hours if you're anything over CIF at 5 FPS. There are a couple of low end consumer grade cellular routers (Peplink Pepwave Surf on the Go comes to mind) but their ability to stand up mechanically in a harsh, full time use environment is questionable to me at best.