Uh, don’t?
You want a Lemmy instance - that I presume you would want to be somewhat reliable - without doing anything? WiFi prioritises convenience over speed and reliability. So, things will randomly fail.

I guess pay for a Lemmy instance provider. Probably the easiest. But this is self hosting, and it sounds like you want a place to start and have chosen “hosting a Lemmy instance” as your learning ground.

Something like cloudflare tunnel will let you punch through a firewall without having to mess with network stuff.
A docker compose stack makes things as easy as they can be in such scenarios.
These are terms you can google “Lemmy docker compose cloudflare tunnel”

Here is 1 result: lemmy.world/post/299429

Here is a GitHub for Lemmy in docker compose github.com/Drakeyves/lemmy-docker-setup

This looks like it covers cloud flare in a compose stack: joelparkinson.me/self-hosting-with-cloudflare-tun…

Read through, learn docker compose, understand cloudflare & cloudflare tunnels