The problem here is that OpenWrt does not have a critical feature required for your setup: automatic reconnection if the operator disconnects the modem. Therefore, to provide the required reliability level, the piece of hardware that connects to the 4G network absolutely must run its stock firmware. You cannot use any modem that is exposed to OpenWrt via AT commands, QMI, MBIM, or similar protocols. It has to be just a generic upstream router connected via Ethernet (or USB "ethernet") and configured to reconnect if the link fails.
See this forum topic for a possible workaround if you disagree with the recommendation above:
I cannot recommend the solution proposed in the thread above because you don't have enough time to test it properly.
Therefore, the recommended setup is to have something like Huawei E3372H in HiLink mode (not stick!) and a router with a USB port. Or MF286D with stock firmware and any OpenWrt router that can handle WireGuard with a sufficient speed.