I recall seeing talk about how Lemmy instances require a domain name. If that's true, you'd at least have to have a static IP if not pay for an actual domain name. I haven't tried to start one up myself though. Mostly because if that is truly a requirement, I wouldn't be able to.
That's why I said or. You need a static address of some kind for Lemmy; be it just an IP or a domain name. I haven't seen domain names given away for free since I was in high school; where would you get one now?
Cloudflare has a free tier for domains, although they're a little sketch. But they also support DDNS, so which updates the IP that your domain points to when it changes