"Upgraded" to Ultimate now house is shut down

Hi, I have had the 150mbps internet for almost 3 years.  We recently "upgraded" to 300mbps and changed to an appropriate modem from the Cox rental to a Netgear CM700.  Our router is a Nighthawk Netgear.

Our internet routinely crawls with speed tests sometimes as low as 30mbps and locks up when trying to stream internet or loses connection completely.

We've reset the router and modem.  I've called technical support where they're advised me to rest.  We've had a technician out who spent a little time inside and did work on pole outside and things got worse.  He left and the internet was not working.  I kept rebooting system and eventually got a weak connection.

Any advice would be appreciated.  There is a definite change and degradation of service since "upgrading."  I've read elsewhere on this forum that settings have been wrong on Cox's end.

Thanks