Perhaps someone will find this useful.
There are some things that could be improved,
- Huawei devices are better/easier to use. Look for E3372 dongles, I personally find the best success with E5776 - they are the most stable.
- There are two modes - stick/hilink for USB dongles. I prefer hilink where USB dongle will have its own dhcp server and web UI. It will give you an API to call for device reboot that is not covered in OPs guide.
- By using hilink mode, most of 3proxy setup can be discarded, it will have only few lines.
- Don't go with Raspberry if you want more than 3-4 devices at the same time, I had some performance problems, old laptop or PC will work a lot better, it will save you some headaches. For testing and small setups - raspberry is fine. A lot of resources mentions that pie can handle 6 devices, but it did not work for me, possible because of hilink mode.
- Get a good USB hub, devices needs power and your raspberry/pc cant hold them all. Be careful by choosing USB hubs as some of them are bad. It needs to have external power. My choice was ORICO 10-Port Hub.
Have fun.