300Mbps Internet speed suddenly only getting 100Mbps, Arris SB6190 modem

I've had the Performance Pro internet rated at 300Mbps for a few years now, using the Arris SB6190 modem and Google Mesh Wifi, it's always been very consistent at 300+ Mbps, the Google router tests speeds daily and I have a log that has the last 30 days being 330 to 350Mbps.  

However 4 days ago on Friday the speed suddenly dropped to below 100, no big deal I thought as it has happened a couple of times in the past, I'll power cycle the equipement.  It got better but stayed around 100-120Mbps, all throughout the weekend and into now Monday.  I've contacted support three times, through chat and phone, they've diagnosed/tested/restarted the modem and said I need a new modem and to get a DOCSIS 3.1.  While I'm willing to go get an upgrade, I just can't imagine the modem is now half working when it's been at full speed for at least 2 years now.

I have not done any equipement or wiring change, can someone help me troubleshoot this further?  Btw not sure if this is related, but a few weeks ago I notced outside the house where the cable connects to the street, there's a new metal cylinder thing connected between the cables, about the size of a finger, the ground wire has also been changed out, it has a green tag on it says Comcast and Do not remove.  A technician must have put it there recently since it wasn't there before.