Upgraded to Extreme-Internet is Now Extremely Inconsistent

Hi, 

We have:

Modem-Arris Surfboard SB6190

Router-Netgear Orbi RBR50

We were having no problems with our internet, but we received upgraded internet to Extreme 300 as a result of getting a new 2 year contract. Now our internet is just unusable. We get fast speed tests interspersed with extremely slow speed tests.  It seems like something might be interfering with the connection since it is so inconsistent and constantly a problem. It is especially frustrating because the internet was fine before the "upgrade." Any help would be very appreciated as we have people doing school and working from home. I have already called Comcast and gone through restarting everything.

Thank you