I did something similar with OpenWRT and a MikroTik box. It works fine. The interesting bits were getting OpenWRT to work on a then-unsupported device (it was very similar to other devices, though), picking a modem with good support for multiple US carriers, depending on the modem, disabling USB3 by taping over those pins, flashing carrier-specific firmware, and getting the modem to connect.
Then I went overboard and set up routing so bulk traffic (OS updates, backups) go over a slower DSL connection, while other traffic goes over LTE. And if LTE goes down, it switches to the DSL connection.
Your comment motivated me to check if I could make my home router fallback to my second connection (I flashed it with OpenWRT). After fiddling with using usb wifi dongle as second wan entry and looking over if any nearby electronic shop has usb lan adapters in stock, I discovered that my router has vlans and I can "rob" one switch port for second wan.
So I set up mwan3 and have more and more respect for what one can do with Openwrt and off-the-shelf router.
Then I went overboard and set up routing so bulk traffic (OS updates, backups) go over a slower DSL connection, while other traffic goes over LTE. And if LTE goes down, it switches to the DSL connection.