I wrote as a Safari Open command prefs://root=INTERNET_TETHERING instead of as above and it almost works. It is all one command, the line break is just the display. The problem is still that while I have the trigger, when the BT connected it prompted me to run the script instead of just running it. When I clicked to run it opened the hotspot screen. At least that proves the syntax is right. The trigger is as above. I didn't do anything with wifi as I don't think it is necessary. My ios version is 14.3 fwiw. Oh, I'm in the US if that matters too. I forget this is a UK thread that I just happened to reply to a while ago and I get alerts.
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Edit: In looking into the automatic trigger, this is something that ios just doesn't do. It is one of the requested items that Apple just won't implement.