Hopefully Useful Tips
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I spent some time yesterday trying to flash UT on my new Pixel 3a and got some info that might be helpful.
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Issue:
- OEM unlock is disabled (grey)
- I was going through the first Android initial configuration skipping all (as I just wanted to check if it worked)
 - When trying to unlock bootloader from Settings menu it was not possible (message: Contact carrier or connect to internet)
 - Setting internet connection through wifi was not a solution
 
 - Possible solution
- My device is not locked by any carrier
 - I had to reset Android OS
 - Insert a SIM when asked
 - Set language and region accordingly
 
 - After this I was able to unlock the bootloader
 
 - OEM unlock is disabled (grey)
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Issue:
- Phone gets stuck on Google logo instead of booting into recovery
- I had tried several cables and different usb ports without success
 
 - Possible solution
- On the UBports installer, tick «Wipe user data»
 - Once the fastboot partitions are done, reboot into recovery mode
 
 
 - Phone gets stuck on Google logo instead of booting into recovery
 
More known issues? Add them here
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Some general Tips :
- Some phones want to do the Bootloader Unlock stuff via the 2G/3G/4G/5G Data Connection.
 - Wiping Data is something you pretty much always do when flashing a Custom ROM or alternative OS on your phone!
 
So that explains the issues you were having

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I didn't have any problems with my 3a after I flashed Android 9 (Sargo).
https://developers.google.com/android/images#sargo
It's very easy to do. During the Flash it gives you the option to unlock the bootloader. After I flashed it, UBPorts picked up my 3a right away and it installed pretty easily after that.
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@geekbone said in Hopefully Useful Tips:
I didn't have any problems with my 3a after I flashed Android 9 (Sargo).
I couldn't flash anything as the bootloader was locked. My device came with Android 10 (or something)
As @nero355 says, I had to check first with mobile data connection
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Thanks for the hint. In my case it was not enough, i got to install Android update (12) and do a factory reset then ( still having the SIM up ), then next run i was able to toggle on OEM unlock.
Note that by installing the required version later, the setting was greyed again but did not blocked the UT install process ( bootloader stays unlocked ).