Not Getting Full Speed I Pay For and Service Drops

I recently upgraded my account from 175mbps to 600mbps. At best, I get 450mbps when I do a speed test, but usually it closer to 350mbps. Plus about once a day, I get a "internet not available" message on some devices, and need to reset the modem. Modem is Arris, less than a year old and was verified by tech to be more than adequate for 600mbps. 

Very frustrating. Why does the service drop off, and why am I not getting close to 600mbps? Further, why am I not getting that speed more constantly? We work from home opposite shifts (day and overnight), and I've also recently gone back to school which is online. We are using (and in need of) the internet constantly around the clock. The kids game and stream most of the day as well, but the main issue is the ability to do our jobs and attend online classes. Really is an inconvenience (and unprofessional) to have to tell your employer your internet is down again.

What gives? How does this get resolved? I'm tired of having to reset my modem all the time.