It is possible for a believer to be in a situation where he cannot obey the government, but he is to submit to it.
Submission refers to our attitude--how we respond inwardly to those who are in authority; obedience refers to our visible actions--how we respond outwardly to those in authority.
For instance, Peter and John were forbidden to teach in the name of Jesus by the government authorities of that day (Acts 4:18). They could not obey these injunctions, because Christ's command for them to proclaim the Gospel superseded the command of the rulers.
However, the apostles later submitted to the punishment that the government meted out and then kept right on preaching (5:18-20). They could not obey the government's commands, but they could submit to the power of the government.
When confronted with their disobedience (v. 28), Peter and the other apostles answered: "We ought to obey God rather than men" (v. 29). We see their submission by their willingness to obey as far as possible.
A contrast between submission and obedience is also seen in Acts 5:40-42: "When they had called the apostles, and beaten them, they commanded that they should not speak in the name of Jesus, and let them go. And they departed from the presence of the council, rejoicing that they were counted worthy to suffer shame for his name. And daily in the temple, and in every house, they ceased not to teach and preach Jesus Christ."
"Submit yourselves to every ordinance of man for the Lord's sake" (1 Pet. 2:13).