contract with AC Horsens. After signing, Lumb said that he could still develop as a player and praised manager Bo Henriksen's style of play which involved attacking full-backs. He made his debut on 16 September in a 3–2 league win over his former club AGF, coming on as a substitute in the 83rd minute for Hallur Hansson. On 26 November, he scored his first goal for Horsens in the return game against AGF. In the 43rd minute, he fired a long-range shot at AGF-goalkeeper Kasper Kristensen, who failed to save the ball, which bounced off his gloves and into the net. Horsens won the game 2–1. At the end of the season, Lumb was nominated for Horsens Player of the Season, but saw himself beaten by team captain Hallur Hansson. He finished the season with 24 appearances in which he scored three goals. He continued as a starter at left-back in the 2019–20 season, and attracted interest from clubs abroad. In January 2020, Horsens manager Bo Henriksen confirmed that Greek club Larissa had put in a bid for Lumb's services, as well as an unnamed Turkish club. Horsens ended the regular season – which had been suspended due to the COVID-19 pandemic – in eighth place, but qualified for the Superliga European play-offs by ending first in their group in the relegation round. They were, however, eventually knocked out over two legs to OB. Lumb made 35 total appearances that season, in which he scored three goals. Horsens started the 2020–21 season poorly under new manager Jonas Dal, who had replaced the outgoing Bo Henriksen. Lumb tested positive for SARS‑CoV‑2 on 14 October 2020. He returned to action on 8 November in a 0–3 league loss to SønderjyskE. Brøndby On 4 January 2021, it was announced that Lumb had signed a one-year contract with Brøndby, replacing regular left-back Blas Riveros who had suffered a season-ending injury. He made his official debut for the club on 4 March as a starter at left wing-back in manager Niels Frederiksen's 3–5–2 formation. He was replaced at half-time for Oskar Fallenius as the match ended in a 0–0 draw. He finished his first six months at Brøndby with only two appearances, as he struggled to make the team. The club would go on the win its first league title in 16 years, which was also Lumb's first league title in his career. Lumb made