Same issue with zbt we-826.
Nothing related with openwrt neither zbt hardware.
Instead is my EC25 lte modem inside the router.
With firmware EC25EFAR06A06M4G_01.006.01.006 working like a charm with 3 LTE networks in Portugal. ( Meo, NOS, Vodafone ). But same router configuration and LTE EC25 with a newer firmware ( can't remember correct version ) Meo ISP SIM card, don't get a valid gateway or return error in connection.
Try to contact Quectel ( if your lte card is a Quectel ) and explain your issue.
The modem module and its firmware controls the link to the 4G network. The SIM card slot is connected to the modem module, not the router CPU. OpenWrt has very little to do with the link other than basic configuration of the modem.
It's usually best to start with APN unset, so the modem uses the preferred APN stored in the SIM card. This works with most modern modems and carriers.