FWIW, I once got the Huawei E173 working with a FEX-20A, v2.0. The main point is to switch the stick from 'mass storage mode' to 'modem mode'. That's what the 'USB string' is for but it didn't work then. Finally, I put the stick on a PC and switched the mode using the supplied mgmt software (? from Vodafone or Huawei, too long ago). After that, it just worked.
Generally, modem stick + FEX is quite a nightmare. But, when it work it works.
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