Hey,
Get the MR3020, it's cheap and works fine. If you want to spend a bit more, and actually it's a bit more helpful which is the MR3040, as it has a battery pack.
What I've got is:
USB dongle, UGREEN from Amazon
MR3020
Anker charger socket
MMI USB cable
3/4G Huawei device which runs CDC/HiLink. Easy to find on Amazon.
I installed OpenWRT version 15.05.01 (whatever is current). Once installed, you need to do a few things (I've assumed the reader of this has good knowledge of networking/OpenWRT):
1) Connect your laptop via a wired connection, it may DHCP to a 192.168.1.0/24 address, but if not, set as .2
2) Set the password on the device, otherwise SSH will not work.
3) Connect OpenWRT to your local WiFi (Network->WiFi) so that you can get some internet connectivity
4) Download putty if you do not have an SSH client (for PC) or on Mac use Terminal, and SSH to 192.168.1.1, user root, the password is whatever you set.
5) Type opkg update; opkg kmod-usb-net-cdc-ether
6) After installing that, which should install some dependencies also, type reboot
7) When it comes back up, insert the 3G/4G dongle in, and on the web interface, add a new Interface called "wan", using interface eth1, set this as a DHCP Client, as it will learn the IP address from the 3G/4G dongle.
8) To check whether this has worked, if you login via SSH as explained above and type "dmesg | grep ether" without quotes, it should show a line with a MAC address, which should look like this: 00:11:aa:bb:cc...
9) Set the Wired port to have the following details:
IP: 192.168.2.1
Subnet: 255.255.255.0
Gateway: 192.168.8.1 (you might need to check what the dongle sets, but this is what mine used and seems to be standard)
DNS: 8.8.8.8
Make sure DHCP is enabled, and on the advanced options tab, tick "force dhcp if another exists"
10) Disable/delete the Wireless adjacency you setup to get internet connectivity.
11) Plug your laptop back in if you unplugged it and change it to DHCP/auto configure, and should give you a 192.168.2.x address. Also, at this point the USB dongle should have a static blue light on it - meaning it's connected and working
12) On your laptop try and visit a website, or 192.168.8.1, and you should see the Huawei page.
If all of that is working drop it in your car, and you should be good to go! I played around with it quite a lot to get it working, so happy to answer any questions.