Can I swap the SIM in a Mobley to a LTE modem with a better antenna?

I have a ZTE Mobley on the unlimited Connected Car plan. Well, its bad, really, really bad. We travel a lot to go camping and use the Mobley to connect to the internet while out of town. We've had a decent connection at times, but most of the times its pretty much useless. I want to upgrade to something that has an external antenna port, a better antenna, or really anything that isn't 5 years old and pretty much useless.

I chatted with someone at AT&T today and he did some network provisioning. It increased our download speed from a .71Mbps to a stellar 2.31Mbps. That was about fifteen minutes ago. I just checked now and its down to a measly 1.88Mbps download and a whole .28 up! And that's using AT&T's hosted speedtest. I checked through speedtest.net and got about the same speeds. 

Now, I want to get a modem that has a better connection because this is crap. I'll buy it, but I want to know if my contract will be canceled for using a device that uses service I'm paying for. Its ridiculous that I'm paying for something that I can't use, and might not be able to do anything about it. I'd love to chat with someone in the Connected Car department or email them so I can actually have it in writing, but well, all they have is a phone number. I don't want to call them, have one person tell me its okay, then have another person tell me its not okay and the first person was wrong.

I'm not being throttled either, I haven't even used half of my 22GB allowed each month. I have also had acceptable download speeds before, up to about 10Mbps and that was incredible. And I'm in an area where I am supposed to be getting LTE speeds according to AT&T's map, but well, this is not LTE speeds.

So what can I do? Get a replacement LTE modem, plug in the SIM and happily use it? Or do nothing, deal with crap coverage on a crappy antiquated device and have useless internet service?