Internet gateway often offline in same time period

I have moved to a new place with my family and had purchased the plan of the gigabit extra service with xFi (25$/month) modem and storm-ready set up. I totally have 50 devices connected to my internet and usually have only 10 of them are active. My mother is a online teacher and she told me the internet gateway frequently offline during between 6 pm to 9 pm. So I took a look and found out that during the offline, it shows no outage around my area and my internet gateway just offline for no reason. It is so inconvenient to have this inconsistent internet and I am so disappointed because I am paying 110$ every month and it can't give me the internet I need. I would like to know what makes my gateway offline, and what can I do to fix it, cause the main reason I purchased this most expensive plan is just for my mother to have a better internet condition to do online teaching, and now it doesn't fit my goal.