If your existing office router or firewall includes a WAN port, and can be configured to failover or provide load balancing via the WAN port, then you can use a 4G router to connect to the WAN port of your existing router / firewall and configure it for failover.
In some instances where you need a public IP address to run services on your network, you will need to use a Fixed IP SIM card and a router capable of Bridge Mode, and use this with a fixed IP SIM card. All inbound traffic to the Public IP address of the 4G connection will be sent straight through to the WAN port of your connected router/firewall.
