What do you want to know? I won't give you 100% of the details, because I'm preparing a 4G telemetry/control + video product for sale.
I also am finalizing an Object Avoidance system using radar. It works with any Mavlink-Compatible flight controller and has a detection range of around 80 Meters, so it can be used at full speed. The Object avoidance system takes control of the drone and makes it STOP, they fly up and to the side (without moving forward), and when the object is cleared, normal flight resumes. So it can be used in all modes - including AUTO. I'll be selling that to Ainstein (www.ainstein.ai) who will use it to fill out their product line.
I'm using a Raspberry pi 4, although a RPI 3B+ or RPI3A+ will work also. I also use a Raspberry pi camera.
Mounted on top is a SixFab Raspberry Pi 3G/4G "hat" with a Quectel EC-25X modem. Here, "X" is dependent on your cell carrier. I use the EC-25A for T-Mobile the EC-25V for Verizon, The EC-25A-F works for both. You will need an antenna for the modem. https://sixfab.com/ I have a 3-6GB/month SIM Card for the carrier. I previously used a USB "dongle", but the "hat" makes for a cleaner installation.
The RPi connects to an Amazon ec-2 server via SSH. The Amazon server is running a "reverse" SSH Tunnel. This costs only a few dollars a month. But I am considering moving to LogMeIn HAMACHI for this function. I haven't made the change yet.
The Raspberry pi is running SER2NET (along with a bunch of other software). This allows for the connection of the Raspberry pi's serial port to the PixHawk at 500Kbaud.
I use a cell phone as a "hotspot" and then - using WiFi, I connect my laptop running Mission Planner to the net. In Mission Planner, I choose tcp as the protocol and point it to the Amazon server and correct port number (9999 is the one I use). Mission planner connects.
I use a hand-held controller that plugs into the USB port of my laptop. This hand-held controller is just like a FrSky or other controller and has a 6 position MODE switch. This is what I use to control the craft.
I bring up CHROME or FireFox browser and connect to the Amazon Server and a different port, and I get 1280 X 720 Streaming video at 30 fps from the craft.
I'm changing a few things to make it easier to setup and use, but that pretty much it.
My object avoidance processor currently handles only one radar unit (facing the front). But it would be easy to add 2 more (front, left side, right side)
And by the way, the object avoidance processor also gives one other function: It adds redundancy to the radios. It channels serial data from the Raspberry pi to the PixHawk, but uses another input for a 433MHz/915Mhz radio. If one source 'disappears' it fails over to the other. It does this more efficiently than simply using another input on the Pixahawk. So the Object Avoidance processor can be used with or without 4G. It also handles any baud rate conversions necessary. The Object Avoidance Processor is a TEENSY 3.5 unit.
I couldn't use the Raspberry for the Object Avoidance tasks because it only has one serial port.