@Tolgahan 

The ER605 has multi-WAN ports, it can connect to more than one modem, or connect to more than Ethernet cable from your GPON / ONT device with a static IP address to get internet. But it can not add the bandwidth together.

Multi-WAN ports router has the function called "Load Balance", the is to help you make good use of the bandwidth, for example, if you are playing games with 1000kbps, the WAN1 port using 800kbps and WAN2 port using 200kbps, according to the Load Balance, next time the router's WAN1 port will use less bandwidth and WAN2 port will use more.

Another function for the Multi-WAN ports router is the Link Backup, if one WAN is down, another WAN will take effect to make sure the LAN is always online.

Sometimes in rare cases, the bandwidth will be added, usually, the bandwidth for both WAN ports is pretty low, just like 10Mbps or 20Mbps. At the same time, there happens to have a large traffic application, then the bandwidth you test is aggregated.

Normally, all the multi-WAN routers cannot aggregate the bandwidth from two WAN ports together.