Setup with 4G passthrough and VLAN

Believe me or not, but I'm familiar with the problem for some years already.
The problem is with encapsulation used by the most modern modems and not with this particular modem. You can easily achieve what you want with ancient Huawei E3372 running in NCM mode - in this case you should be able to bridge your ethernet interface (physical or logical) with wwan, just because this modem uses ethernet framing. You should be able to find some posts about this here.
Found this for you: Using a router as "dumb" LTE gateway?