ANSWER:

You should have prayed asr on your way, on its time. 

You cannot combine prayers while you are still in your city limits. 

I once had to travel after thuhr but just before asr started, the problem is I arrived after maghrib and had not prayed asr yet. If this happens again, and I cannot pray asr here after it begins, could I combine thuhr+asr 4 rakat each here at home before I leave, if I will arrive after maghrib?