Now sorted out. I had screwed up and activated the wrong SIM in the clueless.aa.net.uk website. Then after good staff had sorted me out, I had to wait for ages and ages for the correct SIM to become activated, the servers seemed to be really slow today, and when the status did change to activated on the server, the iPad still wasn’t working. But then I guessed that you have to remove and reinsert the SIM to kick it into life, and that physical reinsertion did the trick.
Reinsertion was a tip Apple support staff mentioned to me.
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So the four grey dots thing means that you can sort-of see the network, but the mobile network operator-related innards are not happy, or the SIM is not activated or forty things.
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Anyway, iPad Pro is working on 3G / 4G now.
This is a replacement iPad because I sent mine off to Apple for repair and a completely different new (still v1) iPad Pro 12" came back instead, but Apple stole my SIM which was a real nuisance.
Also, I made a stupid mistake. I should not have released the IPv4 address that it was using without first changing the firewall configuration because there was an explicit inbound firewall hole for the iPad, as a trusted device, so it could get access into the main LAN over 3G/4G and not dealing with that meant I was letting that IP address get reassigned to another AA user and thus creating a security flaw. Note to self: do not release IP addresses, ever, never ever, not without hunting down references to them first. Perhaps I should have a spreadsheet or something that contains a list of IP addresses or address ranges and then lists all their dependencies, usage, firewall references, dns etc.
I can’t tell if I am seeing any 4G or not, don’t know how to tell. The 4G is, I suspect, pretty rubbish here where I am, lying in bed because of the thick stone wall which just creates a shadow because the angle of the line of sight to the base station is not quite right here on this side if the room. I get 2 bars though and I am assuming that is only 3G, don’t know what the evidence for this is though.