When voting on legislative matters, modern members of congress must act as?

The congress should review the bill initiated by the representatives. The bill will go through a tedious process before approval. The congress will review the pros and cons of the bill and they will vote if it is to be endorsed to the next level or to be amended.

The congress must act as both trustees and delegates

Trustees means that the congress held the trust from the public for their constituents and decision making in the government.

Delegates means that whatever the congress do in the decision-making process, it should be done to represent the will of the majority of the people.