After doing some more research they have an "Acceptable Use Policy" which should is separate from their terms of service. It appears that they do not officially allow any kind of serving or IP listening, which I find extremely strange, given that even their provided router had options for port...
The ISP terms do not mention any specific networking setups, they only disallow piracy, etc. as well as removing their ONT/router. My setup is likely unsupported, but it shouldn't breach the contract.
For the past 2 months I have been hosting a Minecraft server on port 25565 on my home network. The setup was fairly simple, just a port forward on the router (Asus Zenwifi XD5) to the server computer hardwired to the head node, and it worked fine.
Recently, a local fiber ISP expanded to our...