NVG599 not good too for ATT Fiber

3 days ago tech came. Setup my ATT Fiber (install, etc) and he left. He put in a NVG589. It was quick but after 6 hrs he left in the night... problems started. It started to keep getting terribly slow... like 2mbps upload and less than 2-3mbps download. Before I had the Uverse DSL and it was the 18 but it was still way way faster than this. I called ATT tech support on the phone. They told me it was the wrong receiver. I should have got the NVG599. I see from this forum that the NVG 599 still has the same issue. Yes I know fiber is still a new 'thing' then it should not be like this. I hear 'congestion' issues with the NVG599 especially now many homes have multiple devices like smarttv, ipads, multiple family have iphones, LG, Samsung S8 or whatever. We experienced tremendous slowdown with NVG 589 in the evenings when people watch youtube, netflick streaming, etc... if NVG599 is no difference from 589. What is better or we can do ourselves... could the line the previous tech created be the cause? Thanks