Speed down to 90 Mbps

Hello,

After restart my speeds are over 550Mbps but within an hour they will go back down to 90Mbps. They will stay there forever unless the modem is restarted.

I have tested via Ethernet directly to modem and results are the same. I have removed all splitters inside and outside the house and installed a 6dB attenuator.

The issue has been going on for months now. Modem is Arris SB8200. Channel info is below, please help!