If your current router contains a 4G SIM slot, then you do not need anything else but a 4G backup SIM which you can put directly into your router. The 4G backup then runs through your current router and your Internet connection can keep working as usual.
If your router does not have a 4G SIM slot, but does have a USB-port, then the best choice is an LTE dongle. Simply connect it via your USB port and you immediately have use of all the 4G functions.
Even when your router does not have a SIM slot or USB port you can easily use a 4G backup solution. You simply connect our 4G LTE backup router to your current router, ensuring a continuous connection.