5ghz wifi receiving at half speed

I had a first issue with my 2.4ghz wi-fi PC device. It went all of a sudden from 100 to 50mbps. After endless chats with support, they couldn't solve my problem without sending a technician here. The technician himself couldn't solve it, blamed the route and made me buy a 50 dollar device that "extends" my wi-fi signal, so I could basically receive it at 5ghz frequency. This solved the problem for months, but now again my speed is capped at 50mpbs download again. My upload constantly shifts to 50mbps and 120mbps without any reason. All of my other home devices are with fine, stable internet. This AirTies extender was bought directly from ATT.