Slow wired connection all devices

I have been battling this issue for almost 4 weeks... randomly all our normally working computers running on wired connections slowed down. Troubleshooting has been done from my main desktop. Windows 10, Gigabyte network adapter. Have tried everything we can think of. 

 

Our service is the 600mbps. At any time day or night i can check the wifi to my tablet, laptop or galaxy s10 + and get 450-500 mbps. 

Plugging the same laptop into eathernet direct to the modem (TECHNICOLOR CGM4140COM) results in 150-250 mbps service. My desktop noted above is the same. We have tried troublshooting for hours upon hours with and without Xfinity support. 

Things we have tried. 

Different devices for testing all eathernet Cat5e or 6, New cables, disable ipv6, Vpn on and off, network reset on pcs, fresh windows install on desktop, factory reset modem multiple times, using modem in both bridged and normal mode, installing a gigabyte cabaple Wan and Lan router inline, changing MAC address, changing DNS settings, disabling firewalls on both ends, checking and modifying MTU, offloading, packet sizes, checking negotiation on both ends for speed and duplex, checking drivers and trying older drivers for network, restoring to older windows update, checking and dissabling GPU system, testing desktop on neighbors xfinity gigabyte service (900+ on speed tests of multiple types), tcp settings, frame spacing sttings, disabling wifi all togehter, disabling xfinity public hotspot online, changing date and time settings, and more. im probably forgetting multiple things and we are more then willing to try anything to fix this. 

 

The most annoying part is contacting xfinity and having them walk us through the same questions and troubleshooting for a hour that always ends up in the same place.....resetting the modem.....after the 30th time you would think there would be a new option but apparently not. 

 

when we first got the service transfered to this new address we were getting 600-750 at any time. this started 2 weeks after install and has been a DAILY problem