That is much more complex. First your router must support attaching a cell modem via USB. Even if it does the list of supported modems is somewhat limited. Its not like a PC where you can load the driver for each modem the must put the driver in the router firmware.
If your router supports a modem used by your ISP then it is pretty easy to setup.
If you router does not support these device it gets very complex. Many times it is easier to just replace your router with one that does support mobile broadband modems. To hook it to your current router on the ethernet wan port depends on what the ISP is offering. Best to check their store. They might have a hotspot that has a ethernet port. Most are wifi only. You...