You mean phones settings > VPN? I have turn on these two options after your comment to be safe...
View attachment 5928047
The updated list of steps originally made by Gerardo could be something like:
Remove SIM card from the device. DO NOT INSERT IT UNTIL YOU FINISHED ALL THE LISTED STEPS.
Factory reset phone.
Go through OFFLINE set-up process. (skip Sim card and wifi)
Go to Settings > Apps > See All Apps.
On the three dots on the top right-hand side, tap show system.
Search – Device setup
Tap Mobile data & Wi-Fi – deactivate Background Data.
Go back – Then "app battery usage" – Set to Restricted.
Go to home screen and pull screen down.
Press and hold Internet until it opens.
Select the desired Wi-Fi network and connect (here you can already click on "advance options" > metered > treat as metered. If not, do it fomowwing next step)
Tap on gear on the right of your connected WIFI.
Tap Network Usage – select “Treat as metered.”
Go to the playstore Search "NetGuard", install it
Open NetGuard, give it unrestricted battery permissions (so it can run allways) dont worry its super safe.
Tap the three dots on netguard, settings, advanced options and enable "manage system apps"- exit netguard settings.
on netguerd main screen search "Device Setup" and disable the wifi and data communications by taping the little icons next to it.
In the netguard main screen make sure you turn on the buttom/ toggle on the left top corner and agree to all.
To be safer you can go to settings (of the phone) >network and internet>VPN > tap on the fear next to Netguard > and tick always on vpn
EDIT: if you also click the option " block connections without vpn" in the vpn settings you might have problems
Go back to home screen and insert SIM card.
Enjoy using your unlocked device.