Speeds drop from 100 mbps to 1 mbps San Diego

I have had Cox for 10 + years. This year my internet stability has been poor. I was paying for an old package of 25 mbps so I upgraded to 100 mbps. Speeds increased but still randomly drop to 1-2 mbps throughout the day. The had me replace my router. No fix. Now they want me to replace my modem. They said they can't update my modem firmware because it's a "third party" modem. I am asking for a service tech to come out. Customer service is terrible with this company. I'll update if this gets resolved otherwise I will gladly pay for Verizon 5 G or even Starlink at this point.