Why is my subnet mask 255.255.255.255?

It means that your phone is using a point to point link. All your outgoing traffic will be routed to the other end of your link and nothing will be sent to the local network, which is unsurprising as there is no local network.

A network with a subnet mask of 255.255.255.255 puts each device inside its own subnet, forcing them to communicate with the router before communicating with any other device.

An address with a 255.255.255.255 may be one end of a PTP (point to point) tunnel. Check your routing table using a command like netstat -rn. You may see a route to another address from your IP. The other address should have a route to 0.0.0.0/0.0.0.0 (the default route).