Flex box streaming buffering issues

I am new to Flex, I installed it per instructions and my internet package is the up to 400Mbps.  First few days I was getting buffering watching movies via Flex.  I tested the wi-fi speed in the room with the Flex, download ranged from 20-70 Mbps.  On a morning call, First level tech support couldn't fix the issue and scheduled service call for house the next day.  That afternoon, Advanced Tech support called me and within about 15-20 minutes the bandwidth was now in the 300+ Mbps range near the Flex.  problem solved right?  I thought so.  That evening, I accessed HBO MAX through Flex and the buffering was still there watching a video.  I tried access account info through Flex and it locked up.  I then tested bandwidth speed on two different devices near the Flex and the speeds where in the 300+ range, so what is the issue?  I decided to accessed HBO MAX through Firestick, to test a optional streaming source ... not a single buffering all evening.  Seems the flex box has issues with maintaining a stream without buffering.

Any on else having buffering issues with Flex box?