Help - Modem activation without app

I've purchased a new modem to replace my EOL modem (also customer owned), but HOW may I activate it WITHOUT using the app please?

I have an old phone and also just don't use apps. Surely there must be a way...!

Also how do I get the EOL modem removed from my acct after new one is working.

I have VOIP service through another provider so it's rather important my HSI stays on.

Thanks very much for help!