The Bees lost 2-1, with Sergio Reguilon sent off on eight minutes.
Frank said: "It's an incredible shift the players put in to stay in the game. We probably could, should, have got a point.
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They fight, they run and show fantastic character. Today it was easy to roll over, but this group is special they give everything.
There were one or two situations that went against us. The foul (on Mathias Jorgensen) was a clear, clear penalty."
On being dragged into a relegation battle, he added: We will fight and be ready. It's just about winning the next match."