Iphone 12 in Singapore locked by a Vietnamese carrier

I have an Iphone 12 bought from an user in Singapore. The bought date is 07/11/2020.

However, when I tried to insert the sim, the activation lock appeared and could not activate the phone.

I have tried Singtel and M1 but none worked

I chatted with Jon - top senior advisor and he said my phone is locked with a Vietnamese carrier.

This is a very weird situation.

please give me some help with this.