The issue I had with the PIN thing that @AlbertJP mentioned above was that the primary SIM was not able to support 4G/LTE when the secondary SIM had no PIN set. You can disable / enable SIM check in the security settings for your SIM.
So what happened on LineageOS (14, I still have to fastboot to the newer version, been too lazy to do it), was that my primary SIM did have a PIN, but my secondary had the PIN check disabled because it’s a prepaid thing only to be able to receive calls on that number.
The sequence of events on phone boot as I suspect they happened:
- non-locked SIM gets activated, network services loaded, 4G settings loaded (but no 4G available on this card, and mobile data disabled(!) for this card)
- I enter the PIN for my primary SIM
- primary SIM gets activated, network services loaded, but 4G settings can not load, because LineageOS only supports 4G/LTE on one SIM card at a time
– so I ended up with a configuration where the 4G/LTE settings were only loaded for the first SIM, which was not my data SIM, and I could not use 4G (or even 3G, iirc)
What fixed it was activating the PIN lock on the 2nd SIM card, so that it does not get initialized before the primary SIM. However, now I have to deal with another LineageOS bug (minor inconvenience) that after entering one PIN, the lock screen shows for the 2nd PIN, but while you are entering that 2nd PIN, the screen resets upon initialization of the 1st SIM, and your entries are re-set So basically now it’s: 1) enter PIN 1, 2) wait until screen resets, 3) enter PIN 2, and then you can unlock your phone.