The fifth round tie on Monday night finished 1-1 after extra-time and went to penalties, where Forest won the penalty shootout 5-4 with Sels saving from Jack Taylor.
Before then, Ryan Yates' goal had canceled out George Hirst's opener for Ipswich.
Sels said afterwards: "I'm happy I could help the team to the next round. It was the only penalty I was in the right corner. Before the game you study some penalties and every time I took the wrong corner but that was right. I am happy for the team.
"The game was not so good. In the end extra time is a gamble, you don't want to open too much. In penalties one of the goalkeepers is going to be the hero. I am happy. Looking forward to the weekend now."
On playing Brighton next, he added: "We know what to do. The FA Cup is really nice. A few games and you can be in the final."