So after some digging I sorted this out myself. I posted in this thread where I found most helpful info:
and here is my text for completeness:
I've finally managed to flash my Huawei HiLink E3276-150 4G modem with non-HiLink firmware. Like many before me I had a bit of a struggle so here I describe the steps that worked for me as of February 2018.
You can find many guides from around 2013 and 2014 but using the downloads from those days never seemed to work for me - the update process would find the device but not the port, resulting in Error 10. I'm on Windows 10 64-bit which was definitely not available in those days and some people mention that Windows 7 has better mileage with Huawei flashing software so maybe that's the reason.
Anyway, before resorting to install Windows 7 I had success on Win10 by using the latest non-HiLink firmware version I could find, which at this time is 21.491.03.00.00.
(found at https://3ginfo.ru/downloads1462.html linked from https://3ginfo.ru/download59.html)
Note 1: 21.x is non-HiLink, 22.x is HiLink (avoid!)
Note 2: suffix .00 is generic unbranded version
An interesting fact about Huawei flashing is found in the latest HiLink download 22.470.13.00.00 (found at https://3ginfo.ru/downloads1102.html) where a pdf contains instructions about verifying the existence of an RNDIS device in Device Manager.
If you don't find this device when the modem is connected you should install the Huawei driver/service, f ex from http://modemsolution.com/wp-content/uploads/2013/12/Huawei-HiLink-Drivers.zip, or from the bundled CD partition on your modem.
My successful path for flashing was:
- disconnect from my normal wifi (conflicting with 192.168.1.1)
- install the driver/service from my modem and connect it
- surf to http://192.168.1.1/ and open device info
- verify hardware is "CH1E3276SM"
- note IMEI (needed later)
- I then entered manual mode and disconnected 4G (not sure if needed)
- verify RNDIS device in Device Manager
- start firmware update
- when asking for flash password enter the result of IMEI to Flash Code calculation at http://www.mobilabredband.se/huawei-unlock-code-generator.php
- done and all fine (modem now working great in NCM mode plugged into my OpenWRT/LEDE router)
BTW, for reference I will just mention that many guides point to the E3276 software at http://www.ofmodemsandmen.com/downloads.html which is no longer available. Looking at an old version of the site using the Wayback Machine (https://web.archive.org/web/20150308164626/http://ofmodemsandmen.com:80/downloads.html) shows that it actually points to the E3276 firmware at https://www.elisa.ee/UserFiles/blog_uus/failid/E3276%20W8/E3276Win8.zip which is still available. This is not the latest version any longer though.
Some guides also mention http://dl.mymodem.in/noroot/E303HiLink11.010.06.02.910.zip which is also no longer available. I found it at https://routerunlock.com/firmware-upgrade-for-huawei-e3276-e3131-e353-e352-modem-dongle-download/ and a good thing to know is that it is password protected with "NiRaV" which is also mentioned in a readme file.
AFAIK these two files were not part of my successful procedure but like many other I was trying all different combinations of installs and failing at first, so who knows if some side-effect got things working. Either way the E303 file immediately crashed after entering the password so it didn't seem to get very far.