OK, this may not seem that hard. After all, it is networking. But the mobile networks make this a nightmare, so finally being able to actually ping my iPad is a huge step forward.
We finally have data SIMs that have unfettered IP connectivity (if only for a single static IPv4 address for now). No session tracking. No firewall. No NAT. Just raw IP. Networking like what it should be, in'it.
What is fun is that even when "turned off" (well, the blank screen mode one normally leaves it in) it seems to maintain the data connection indefinitely. And you can ping it! It does take a few seconds to respond initially so I can only assume the radio side is shut down to something that exchanges data every few seconds at most - though once you get through all the queued pings reply and it works.