Some ISPs will block port 80 or port 22. I have a static IP block and had to talk to 4 techs before one agreed to open up 22. It's just a matter of monies for them. If I paid for a business package they would have unblocked it right away.
You can get around a port-block with a service like noip, but you will not be running a server that will be shutdown, or at least you won't have a community to work with if the service is not available around the clock. You shut off the power, the service is down. No way around that one ?
It appears there will be at least a couple of hosting providers for Flarum in the coming months ? ? While we are not going to be free, the price will be quite reasonable and worthy of the small investment.