All LTE modem cards use USB internally. So why opening up a device for no reason when you can use the available USB port to easily add a new modem.
For the records, only Linksys WRT1900AC v1 has mPCIe, and you would need to cut the heatsink. It’s so much work, and not worth the hassle.
If you don’t find a suitable USB modem, I could suggest some affordable external Huawei LTE modems instead. Something like second hand Huawei CPE E5186, and buy it some external antennas. It’s a Cat6 LTE modem and can support speeds up to 300Mbps. You can enable bridge mode, as well. You can find them on EBay for less than $50.
Other Cat6 models could be an alternative. They can easily fit a waterproof encasing.