Netgear modem/router combo questions

Netgear Nighthawk C78000 to be specific. Purchased June 2021 and suddenly won’t keep a steady internet connection. It keeps cutting in and out. Unplugging sometimes helps but not always. Cox says everything looks good on their end and is blaming the Netgear. I do have a Cox-installed splitter that I’m unable to disconnect and they think it could also be a splitter problem. Tech coming out tomorrow. In the event that the splitter is not the problem, any recommendations? Is it normal for a Netgear to only last 2.5 years? What does a splitter even do? If I’m forced to buy new equipment what Cox-compatible combo units are recommended? A lot of questions, so thanks in advance!