Many people are maintaining that Trump is not their President.
Actually, they are correct - he's not their President, and he's not my President, either. Why?
US Representatives are chosen by the people whom they represent. So you can correctly say "he/she is my Representative".
US Senators originally were chosen by the legislatures of the states they represent. That got changed to direct election by an amendment which weakened the power of the states. So you can correctly say "he/she is my Senator".
But the President of the USA is chosen by the Electoral College, and he/she is the executive authority of the USA., which is an entity separate from the people themselves. Therefore, you cannot correctly say "he/she is my President".
Haven't you noticed that his oath concerns the Constitution and only the Constitution (i.e., NOT the country, NOT the people, NOT the economy, etc.)?