the context given was: "Kamerad Müller hat gesagt, dass..."
in this case the rank must be used, eg., "private/corporal/sargeant Muller said..." - there is no other acceptable title to use in addressing a soldier by his name, as far as i know.
"comrade" would never be used, because of its association with communism. before the dissolution of the USSR, it would have been a strong insult, implying the soldier was a traitor to his country, and sympathized with the enemy. one exception however, is the term "fallen comrades", which does not carry an association with communism.
although soldiers may talk about "fellow soldiers", it would never be used as a title to address someone, as in "fellow soldier Muller said..."
as well, the following are plainly incorrect:
"soldier Muller said..."
"buddy Muller said..."
"companion Muller said..."
"mate Muller said..."