I just got a new Galaxy S8 and used by mistake my Fido's SIM card on it. The phone restarted, installed Fido apps and now it's locked to Fido. Any way to resolve this, I'm kinda desperate here.
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Welcome to the community!
Samsung phones locks to the first SIM that you put in the phone.
If the S8 is brand new and the Fido SIM was the first to be inserted in the phone then the phone will automatically reboot change the CSC to Fido and lock itself to Fido.
If you want to use the phone with a different carrier you will need to get it unlock.