On 12 July 2017, Mattioli and Michele Di Gregorio were loaned to Serie C side Renate on a season-long loan deal.[2] On 30 July, Mattioli made his professional debut for Renate as a substitute replacing Francesco Finocchio in the 89th minute of a 3–1 home win over Siracusa in the first round of Coppa Italia.[3] On 3 September he made his Serie C debut as a substitute replacing Gabriel Lunetta in the 79th minute of a 1–0 away win over FeralpiSalò.[4] On 29 October he scored his first professional goal, as a substitute, in the 77th minute of a 2–0 away win over Bassano Virtus.[5] One week later, Mattioli played his first match in Serie C as a starter, a 2–1 home win over Gubbio.[6] On 9 December he played his first entire match for Renate, a 3–1 away defeat against Fermana.[7] On 25 March 2018 he scored his second goal in the 77th minute of a 2–0 home win over Vicenza.[8] Mattioli ended his loan to Renate with 32 appearances, 2 goals and 2 assist.[9]
Loan to Cuneo and Südtirol
On 20 August 2018, Mattioli was loaned to Serie C side Cuneo on a season-long loan deal.[10] On 17 September he made his debut for Cuneo in Serie C in a 1–0 away defeat against Pisa, he was replaced by Davide Arras after 67 minutes.[11] One week later, on 23 September he played his first entire match for Cuneo, a 1–1 home draw against Gozzano.[12] In January 2019 his loan was interrupted and he returned to Inter leaving Cuneo with 16 appearances, 14 as a starter.[13]
On 31 January 2019, Mattioli was signed by Serie C club Südtirol on a 6-month loan deal.[14] On 2 February he made his debut as a substitute replacing Tommaso Morosini in the 90th minute of a 4–2 away win over AlbinoLeffe.[15] On 28 April he played his first and only entire match for the club, a 2–0 away defeat against Imolese.[16] Mattioli ended his loan to Südtirol with only 6 appearances.[9]
Modena
On 19 July 2019, Mattioli joined to newly promoted Serie C club Modena on an undisclosed fee.[17][18] On 25 August he made his debut for the club in a 0–0 home draw against Vicenza Virtus, he played the entire match.[19] He became Modena's first-choice early in the season.