C7000v2 Speed Issues

Thanks Furry 38.  I do not have the Killer Ethernet Card.  Only have the Netgear AC1900 C7000 Version 2 Cable Modem Router.  ISP deliver speed showed down-speed of 250-300 mBS when the Spectrum Tech checked the line going to my C7000V2.  Once he reconnected the line to my C7000V2 last night, I was getting 200+ mBS at my laptop and on a phone.  20 minutes later the down-speed signal strength down-speed dropped back to 1mBS.  Figuring I need to replace my modem router.  But may try  a Toner Cable Co RPA42-Return Path Attenuator at the cable to modem-router connection if I can determine the correct dB level to order.  There are 3, 6, 9, 12 & 15dB attenuators.