I also have exactly the same device. I have tried to unlock it by uncoking the router and soldering the wires on the motherboard of the router, but it wasn't recognized by on my Windows 10, despite having installed drivers. So im also having to resort to looking for the unlock code alsewhere. I'll be glad if I could get it here.
The IMEI is 866175028549064
S/N: F5S7S15A10010339
Mine is B310s-927, I too went the same lane and got this same response. Well, The issue isn't the Windows 10, rather it's the 21.311.xxxx software version of the modem. I later paid £8 to unlock it online.