Where is wrong? Can you provide a screenshot?

Confirm with your ISP that OTE S.A.(ISP) is not using VDSL connection. Go to Network---DSL Settings and change DSL Modulation Type. Try ADSL2 or G.DMT.

If you just want to improve Wi-Fi signal, it would be better to buy a range extender and add it in your network.

adiSal wrote Yes, whilst connected directly to the internet, the games run and I experience a noticeable decrease in latency in all the above mentioned games. From 200 ish in Diablo to 89 , from 170...

Hard to say... Check the hardware verison and firmware version. If there is no newer firmware, replace it under warranty.

Can you try to install the utility on a different computer? Disable anti-virus software and firewall. How about status of all light on powerline adapters? In some cases, even if they are not detected...