@TP-Link good morning,

Did 2.4ghz and 5ghz have the same issue?

--if yes, Have you tried to enable the high speed mode, such as 5gzh to the main router, 2.4ghz to the client;

Yes, and happens the same. With the 2.4 ghz is more stable but is a little bit slower. (anyway, the problem still happens)

--and please also try to change the wireless channel on the 5ghz of the main router, such as channel 40 and 48; channel 11 for 2.4ghz;

I've tried 36. 40, 44, 48 and always happens. The 2.4 ghz is currently set to 11 and the 5ghz to 48. I will let you know in a couple o f hours.

And if possible, please also check the Ethernet connection from the RE450 to  see if the Ethernet connection has the same issue as well?

Ok now i'm connected with the ethernet (and 5ghz connection provided to ethrnet port). and the connection works and suddenly stops for 10-15 seconds, in random way.

@TP-Link 

I can confirm, it worked for 2 hours and now is stuck again, it doesn't get the ip address and can't load the pages 

@TP-Link I've tried to set the High Speed mode (both manual and automatic) but it gets stuck after a while. The same issue i had before.

Now i'm resetting the extender, again, and try to connect to my router by WPS button.

@TP-Link nothing.

I've done everything.

I even tried a new modem but the problem still happens.

What can I do, considering that i've payed almost 60 euro for this repeater and it DOESNT work?