I don’t want to be rude but if you can’t afford a domain you probably shouldn’t be hosting a fediverse server.
Honestly do you even need to expose services to the internet? Internet exposure is dangerous and is not necessary for 95% of things. You can use a mesh VPN like netbird or Tailscale if you need remote access.
This isn’t me trying to offend you I just think it would be wise to reduce the scope of you projects.