I would imagine port 80 is being blocked by your ISP for your protection. ISPs often don’t give you a fixed IP address so even if you get this working it might suddenly break one day if your public IP address changes so it would be worth checking this in advance and considering hosting online if you haven’t already.
If you want to test if WordPress is the cause of the problem simply point apache at a different folder containing a basic index.html with some plain text in it, does that work? or same result?
If you want to test that port 81 is open use nmap to hit your IP (from a computer that is external to your network) nmap ip-address-here
I’m presuming you know this but I’m going to say it anyway, make sure your domain is pointed to your external IP not your internal IP (i.e. the IP that you get if you google “what’s my IP”, so nothing starting 192.168.0.X)
Basically narrow down the problem and check that it’s actually a WordPress configuration issue because otherwise it could be any number of things. If it’s not a WordPress issue you’ll probably have better luck posting on another part of StackExchange like Server Fault
But in principle no you’re not missing anything, point domain to your IP, allow exceptions in any firewalls and setup port forwarding, tell apache what port to listen on, tell WordPress what domain/port it’s on.