Worse internet speeds after upgrading from 100Mbps to 300Mbps speed plan

Hi,

I've upgraded from the 100Mbps to the 300Mbps internet plan on 8/7/22 and I've only received worse speeds as a result. It varies so much throughout the day but more often than not, its worse than before. Previously, it was at least consistently decent speeds but now its anywhere from 2Mbps - 90Mbps, and has never broke 100Mbps. This was tested on my desktop PC that I've been using just fine with the internet for the past 8 years. I've attempted restarts of the modem/router but its actually to the point where it even has issues connecting to the internet sometimes. Is there any way to investigate the reason for this? Why am I paying more for worse?

Modem: ARRIS SURFboard SB6141 (supports up to 1Gbps)

Fast Plan

Download speeds: up to 300 Mbps

Upload speeds: up to 10 Mbps