Internet Service Provider Options - Avondale at Warner Center - West Hills

I have a client who lives near Randi/Victory (West of Topanga/Victory) and he has an unsteady internet connection (Spectrum). According to speedtest . net, his speed can be as high as 300mbps but seconds later he's having trouble with zoom/IG. I know that I can look up service areas for Frontier, ATT, and Verizon, to name a few, but I'd rather hear from people who live in the area. Which ISP do you use and do you recommend it? I have already tried different routers and different modems but the issue persists. TIA. I'm hoping this community is large enough for someone in the area to make a good recc.