I did a factory reset and nothing.. still missing those 20Mbps, is it dying already my router?

@FiruzzZ 

First thing I would check is the Ethernet cables as if speeds are less than 100, that may be the issue. 100 and less use 4 wires and above that require 8 wires. Also you need CAT 5e or better.

Some PCs change the speeds when connecting to different routers/modems so check your PC drivers and such.

Are you using a VPN as that can slow things?

Are you using QoS as that causes issues?

Are you using any router security?

Is the router running hot?

Is the firmware up to date?

Some routers have connection speed setting so check that also.

Are you using both IPv4 and IPv6?

Is there anything that can be taking your bandwidth such as using cloud backups?

Did you change DNS servers?

Ethernet speed is 1Gbps, no VPN, QoS is mandatory in this shitty router (it is in standard), no router security (no antivirus, no firewall, no access control) config just a few ports forwarding, the router is always hot at least for this model, no new firmware released, just IPv4, I tested the connection in complete isolation and from every device that I have (pc, notebook and phone), didn't change the DNS, using the ISP's. It must be a hardware degradation (in less than a year amazing)

@FiruzzZ 

Contact support as it has a 2 year warranty.