I don’t know if anyone has murmurings this yet, but if you leave a landline that is through your ISP, and connected to the router, you may not be able to use your own router. Spectrum doors not let you, and despite all the research I did before, I did not find that out until after I purchased my own. You can ask them to split the line and use the old router with the phone, and your new router for internet, but there are potential issues with degradation of service involved, as well as the more mundane problems of running into ISP support who are not interested in understanding what you want, and getting the line actually, physically split.