Yoane Wissa had the visitors ahead inside the first minute before City fought back to win 2-1 thanks to Erling Haaland's brace.
Frank later said: "Standing here I am a combination of extremely disappointed we didn't get something out of this game but on the other hand very proud.
"The first 25 minutes was world class, we were better than Manchester City. Second half they were more on top and made it difficult. A point would have been fair and not often teams can come here and say that.
"You could say if we need to win here we needed to take more of the chances we had in the first half.
"I respect Pep Guardiola very much and a lot of the reasons I do is because he made tactical changes at half time and then he is not too proud to put the ball down in the corner because they need to win. We must have done something right today.
"This is a fantastic group to work with and we need to keep doing this.
"Haaland is a fantastic striker, he got a little bit of help today with a deflection and a mistake we don't usually do."