OK, now I’m starting to understand. I think we should clear some things up.
With respect to privacy, creating your own subnet and firewall inside your apartment will do very little besides buy you a false sense of privacy. That’s because, as soon as your packets cross from your apartment network to the school’s wireless network, all the packets will be decrypted (or encrypted with a password everyone knows) throughout the school’s network. If you are worried about snoopers, your best choice is to use a VPN service. This will give you a strong, encrypted data pipe to a point outside of your school. If you (or a friend) do not have a server outside the school you can configure, you are best to use a VPN service like Private Internet Access (just one example, there are many).
As far as security, it is true that a subnet and firewall makes it hard for others on the school network to attack your devices. However, I suspect you spend a non-trivial amount of time with some of your most precious devices in and around the school, where the firewall in your apartment will do little to help. If so, perhaps it is best to rely on other forms of security. Use strong passwords and a password manager like LastPass or KeePass, and/or multi-factor authentication (e.g. YubiKey). Turn off devices when not in use, or at least turn off wifi to keep them off the school network when it is not needed. And never leave a device unattended. Physical access to a device almost always undermines most forms of security. And if you really want to keep some devices (e.g. a desktop computer) behind your own firewall, just place it close enough to your WISP device and don’t bother using it on the other side of the apartment.
Sorry if this is taking a different turn than expected. If you already have excellent, free wireless throughout your apartment, that is definitely a special use case. I’m not sure an extra B1300 is really what you need.