PeeringDB

The Interconnection Database

I switched to LTE and it worked fine, so I suspect there's a connectivity issue between my ISP and your folk's service.

Unfortunately, IME, chances are that this isn’t an accidental / peering problem. A lot of ISPs are blacklisting domains that have been classified as "malicious" or "phishing" by somewhat trigger happy security companies. Also, even before checking with your ISP, assuming that you don't have a dedicated firewall or internet security software, do have a look at your router admin console if you have access to it. Nowadays, most routers have built-in home network security functionality. For example, Asus routers have "AiProtection", Netgear routers have "Armor", TP-Link has "HomeShield" and so on. I believe most of them have their security services enabled by default out of the box. They all essentially defer to a "select" group of security companies to identify threats (Trend Micro, Bitdefender, etc.), and these companies often copy each other’s homework. So, it would be interesting to check if jellyfish.land is not being flagged as a "malicious website" and blocked by one of these mechanisms.