I recommend using a router with WAN failover. With this setup you will need 3 devices:
- Router w/ WAN Failover
- DSL/Cable Modem
- 3G/4G/WiMax Modem
While perhaps more costly than a integrated device, this will provide you with greater freedom in case 4G comes along or you switch primary internet providers. Also, in my experience, a dedicated office router can yield additional benefits such as better VLAN and VPN management.
Cisco and other brands have various routers/gateway devices with WAN failover built in. Many Cisco IOS devices can have WAN failover added to their configuration.
There are also software based solutions like:
You have not mentioned what services you are trying to access? But with these devices, your public IP address will switch and any persistent connections will drop. So any services that depend on a persistent connection or specific IP can be impacted.