@Newtonetwrok 

Make sure you're using the original cable from the C6 kit to connect to the wall socket.

Check the negotiated speed of your Internet port here:

If it's less than 100 mbps then try changing the cable that connects C6 to the wall socket with a new one (Cat 5E/6) and observe that speed.

If your main ONT router is gigabit capable and the devices (ONT router and C6) are negotiating properly, the C6 Internet port speed should indicate 1000 mbps.

@terziyski 

The router comes with Cat5e cable in the box, I tried connecting with Cat6 too.

The negotiation speed out of the box was at 10 Mbps full duplex, if I set it to 1000 Mbps full duplex it doesn't change, but when I set it to 100 Mbps full duplex, the speed shows as 100 Mbps but my speeds are still at 20 Mbps max.

@Newtonetwrok 

Maybe the wall socket is the culprit for your issue.

If you can connect C6 directly to your ONT router (bypassing the wall socket) to confirm what's the negotiated speed in that case, then you'll know for sure.

At first you can try with Auto, then try 1000 mpbs full duplex, then 100 mbps full duplex to see wich speed they have agreed upon.

@terziyski 

I tried connecting my PC directly to the wall socket, the speeds are 98-100 Mbps, but let me try connecting the router by bypassing the wall socket