Spurs won 2-0 in Frankfut thanks to goals from strikers Randal Kolo Muani and Dominic Solanke.
Frank said afterwards: "I think, first of all, very happy for the performance. I think the players showed great character, their mentality. We basically had 12 available players for Dortmund and in this game and in both games the players stepped up.
"And I think this game we were more or less in control from the beginning to the end of the game. I think that's a big thing. I need to say, we've seen some of the results in the Champions League tonight. We've seen some of the results throughout the league phase. There's no easy games. So we're sitting here with 17 points, ending, finishing fourth in the group stage, sixth sheet.
"I think it's very impressive, but throughout the game today I think we were good in the high pressure. I think we looked strong when we defended low. I'm very happy how we progressed with the ball and how we got quick up the pitch. And then, of course, still working on creating, but I think we created quite a lot today, which I'm very pleased with. So, yeah, very, very happy with that.
"And then, of course, the fans I think were fantastic. Very happy, so many of them travelling in numbers and the support throughout the game."
On guiding Spurs into the round of 16 at first attempt, Frank continued: "Yeah, of course I'm happy, but it's not about me. I think I'm very happy that the staff, the players, everyone, how we prepared the team for these games in general, I think is very positive.
"And I think, of course, we need to look forward to the round of 16, which is hugely important that you are qualified for everything, but also gives us some time between games to train. And I also think, let's say, the 3-4-3 also looked good in the last three games."