SSH connection just sits there without actually connecting or refusing?

Weird one here. I have a Cisco 819 that will not let me SSH into it over the 4G connection. However, I can SSH into it through other interfaces. The 4G is up and working, I can ping the IP of the cell interface, and send traffic through it, but I just can't SSH into it.

When I try to connect, it just sits at the black screen and nothing comes in. It doesn't refuse connection, it just does nothing. I've looked at the config and it's fine, and my crypto keys are fine because I can SSH with the other interfaces.

The router is not even "seeing" the connection.

Line User Host(s) Idle Location 3 tty 3 Async interface 00:00:06 * 10 vty 0 XXX idle 00:00:00 12.X.X.X 

Line 10 is from another interface, but when I try to connect through the cell interface, "sh users" doesn't add another connection. I've tried it on a different router and it does show another connection when I SSH from the cell interface.

I've rebooted it, and updated the iOS too.