For for sim unlocking or bootloader unlocking Acer iconia a500 or any other model of lg Nokia Motorola unlock. You need to follow below steps. Normally when you unlock any mobile like lg, Motorola , htc via bootloader unlock process will be same but require software are different. So you can try this method for Nokia lg at&t mobile sim unlocking.
To sim unlock mobile via bootloader you need following thing for that. Rooting your mobile, cpuid, sbk, usb driver flashtool, fastboot, adb tool and unlocked bootloader as per your mobile tablet model. Sbk can be calculated from cupid.
Some mobile company provide unlock file but you need to provide device id of your mobile to them. Lg provide unlock file for few model even Lenova Motorola provide unlock file for specific model only. Also htc unlock file are available on many sites
You first download software developer kit and install on your computer so fastboot and adb are included in it and some of mobile easily root sdk kit.
First take backup of your mobile before doing any process.
Now download USB driver for your mobile as per mobile model and install in on your computer. Now go to developer option on your lg , Samsung or Acer mobile and turn on USB debugging this should be done when you install your usb driver.
Now download rooting software package as per your mobile model.
And install it on computer.
Also download superuser su and install it. Also download unlocked boot-loader package for your acer mobile.
Download adb and fastboot or sdk kit.
And any good flashing tool for your mobile. There are many free software available for flashing of Acer lg Samsung Nokia.
Now find CPU id of your mobile or tablet. You can use fastboot oem command for this or there are many ways you can find cupid. You can also find it at system/firmware/fuse/acer cupid on your mobile phone.
Sbk will be generated from cpuid. Many flash-tool automatically generate sbk when you provide cupid.
Now start flash tool and follow the instruction provide the path of package that you want to flash and after finishing your mobile is unlocked.