Periodic reboots are actually a PCI requirement for payment terminals heh, basically every point of sale on the market reboots every 24h.
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I am surprised you write that the SIM is locked upon reboot. What does this mean in technical terms? Do you mean that you have to enter the SIM PIN when your phone reboots?
Yes.
I can cancel the popup and then still use the rest of the phone, but when I do that I have no cellular network access until I unlock the SIM itself.
For the avoidance of confusion, the SIM PIN is independent of the phone PIN.
The solution is to use an eSIM - you can disable the PIN at bootup because it is protected by the phone lock anyway and will be operational immediately.