Wellen played for the first team ofCrefelder HTC since he was 16 years old.[4] In February 2020 he signed a two-year contract atPinoké in the Netherlands from the 2020–21 season onwards.[4] During the 2021–22 indoor season he scored the winning goal in the championship final againstHDM as Pinoké won its first national title. He was named the best player of the Dutch indoor season.[5] At the end of the outdoor season he returned to Crefelder HTC.[6]
He was the Player of the Final Match, Best Forward Player, and the Player of the Tournament in theFIH Men's World Cup 2023 held in India.[9] After the2024 Summer Olympics, where Germany won the silver medal, he announced his retirement from the national team.[10][11]
