"Generally a servant is a person who works in a house or an establishment". He is given wages for the work that he does.
An attendant is one who attends on a person or persons. He is a helper, an assistant.
There is a sense of subservience in ‘servant”. This is generally absent in ‘attendant’.
VIPs have attendants. There is a certain respectability about the term ‘attendant’. A doctor is called a medical attendant.
The word ‘servant’ also acquires respectability in certain contexts. In fact, till the eighteenth century ‘servant’ also meant a person who was totally devoted to the service of a lady. Such servants are there even today! But it is not one of the meanings of the word today.

Also when a person totally dedicates himself to God, we say ‘He is a servant of God