You can have issues with self hosting on a residential connection if your provider gives you a CGNAT connection and not a dedicated one. With CGNAT (Carrier Grade Network Adress Translation) your line is shared between multiple users. So are the ports, so imagine you’re trying to open a port on your router and expect to reach from internet to your home using that port. Your carrier may have splitted the whole available ports pool between the 4 customers behind your IP, and when you try to reach <your IP>:<the port> your packets may go Somewhere else. This thing is ok for when you use internet for classical browsing, not when you try to have traffic initiated from internet up to your home
Honest question : Why Mastodon ? I had a single-user instance very soon after discovering the fediverse, but the needed stack to run a Mastodon instance is really insane compared to something like Akkoma / Iceshrimp. I would never use Mastodon for a low number of users, including all the limitations compared to other micro-blogging activitypub compatible services.