Using a gray market phone

I bought a brand new xiaomi smartphone in 2016. But the next year after I checked on government regulations website, my phone was "indeed" proven illegal. Illegal here means imported without any permissions from the government (not stolen, I already know the rules about stolen items). So no doubt about this. This leaves me questions:

1. What is the ruling for anything that we bought, we didn't know it is illegal at the beginning, but then it is proven illegal afterwards? (again, illegal here = "avoid taxes" not stolen items)

2. Am I allowed to continue using this or should I get rid of it?

3. What level of investigations should every Muslim do for each business transaction like this?