I have a Raspberry PI 3B (running Raspbian Stretch). I have it tethered to an iPhone 6 via USB/hotspot using a Mint Mobile SIM card. This setup worked great, but I only tethered the iPhone for RPI and had to reboot the iPhone every now and then (actually it didn't happen that often, maybe once every 5 weeks). I set up the RPI as a WIFI hotspot for a security camera in a remote location where there is no internet other than what is provided via the iPhone and the data plan I had there. So the internet service via the iPhone was the only way for the security camera to communicate with the world (with me) and for me to communicate with it (with reverse SSH - for those who want to know). This worked well but was a problem when I had to restart the iPhone, for example due to a power outage that caused the battery to drain. I had to drive to the location, restart my iPhone, re-enable the hotspot - you know, it was painful. I figured there must be a way to use my Mint Mobile SIM (which is on the T-Mobile network) directly with the RPI. This USB modem does the job. I saw another review that said it works with qmicli (a command line interface for configuring a Qualcomm based USB modem). I haven't figured out what works with qmicli because I couldn't find instructions right away. Luckily I found another blog post talking about configuring this particular modem to RPI. All you need is your carrier's APN. In my case for Mint Mobile it was "Wholesale". I can't post the link so find the instructions this way: search for raspberry-pi-with-huawei-e397u-53-dongle-metro and you will find the instructions. Once you get the instructions, it's wonderfully easy. At the end where it explains how to update DNS service I replaced it with Google DNS which is 8 8 8 8 and 8 8 4 4. Everything works great, I can get 5-22Mbps up and down which suits me perfectly (and considering the device is in a bad T-Mobile range I'm happy with that). Now when I restart the PI, the internet reappears - no iPhone required :)
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