Slow and unreliable internet

I am currently on the plan for 100 mbs. However for the past week or so my internet has only been holding around 20-30 mbs on a good day, and as low as 1 mbs on others. I routinely reset the modem provided to me, and there is only 1 device connected to it, my Xbox, and I've hard-wired it via ethernet cable. I routinely lose connection every other hour, and never reach speeds close to my plan.
I called support the other day and the only progress made was getting it back to 30 mbs rather than 1 mbs.