Use a VLAN aware switch to terminate the 5 ISP circuits. Each one goes into a port assigned to a different VLAN, untagged. Configure an uplink interface to pass all five VLANs tagged. Plug it into the RouterBOARD, and configure five VLAN interfaces on the port connected to the switch. Then run either directly IP the VLAN interfaces if you have modems doing the PPPoE part, or configure PPPoE clients on the VLAN interfaces.
You can only bond if the other side supports bonding. Does your ISP support bonding?
If not, use the wiki on PCC together with what I posted previously. You will need a VLAN aware switch.
I can post more details on either method if you need it, but neither works out of the box and has requirements (ISP support, or a VLAN aware switch).