Many ISP nowadays use their own modem/router with their own firmware. They wont allow you to replace it with 3rd party equipment. As they have custom firmware you don’t know if they are examining your internet usage. What is the best way to prevent them from spying on your usage whether it be email, peer to peer etc.
It will be the case that you can use your own equipment you just need to know the correct setting to establish a connection. When they say “allow” they probably mean it is not supported right? It wont be a criminal offense unless you are in untypical country.
Really I wouldn’t go with a provider that doesn’t “allow” you to use your own equipment on a connection you are paying for. How do small business cater for there more specialist needs!?
As for preventing spying. I create Volume 3 as an answer to that. You use anonymising services depending on your needs.