Update up to iOs 12.4
1 - Position the UMS Sticker alongside a SIM card belonging to the network carrier you wish to use your iPhone with and wait up to it shows the automatic setup menu. Select Manual via IMSI/TMSI.
* If the automatic setup menu does not display follow this route: Settings - Mobile data - SIM Apps - Select Unlock Mode - Manual via IMSI/TMSI - input imsi.
2 - Under EDIT IMSI you will be asked for a 7-digit code. Type the IMSI of the original network carrier of your iPhone
If your iPhone is locked to AT&T, type the IMSI for AT&T even though you are going to use the device on another network.
Click here for the list of IMSIs.
3 - Once you've typed the IMSI for the original network of your iPhone press Send.
4 - Select the TMSI 2G/3G/4G (Auto/Manual) option.
5 - Select the AUTO MODE1 option.
6 - You will get the following message: Setup OK! Restart your iPhone now.
* If the previous steps did not work, please follow these steps with any SIM card with a factory-unlocked iPhone.