Your instance has to be accessible from the outside world for it to federate (remote instances push new posts to your instance, it’s not your instance polling remotes for new posts)
As said in another comment, federation requires HTTPS, which itself requires a domain name. You can probably do it for cheap with a Dynamic DNS service (for the domain name) and Let’s Encrypt (for the SSL certificate required for HTTPS)
lol, Sony came up with that idea 10 years ago, and it flopped hard https://en.wikipedia.org/wiki/DSC-QX100