Slow Internet Speeds

I have a 1200mbps plan and have routinely slowed down to 100-200mbps in the evenings with uploads as low as .22mbps.  I work from home and this is interferring with my ability to use my work phone.  I have repeatedly called and am constantly told that I can purchase a high speed plan or an extender (my computer is wired to the modem). ALL of the routine checks have been completed.  It's not the ethernet cord, modem reset/reboot/replaced.  Speeds exceed 1200mbps in the morning hours and slow way down by the afternoon/evening.  There has to be a way to have more reliable service!