According to the official information brochure https://view.mahacet.org/mahacet/admin/news_document/FinalNEETUG%202020%20Informationrochure04112020.pdf, you might not be eligible for the mop up round. The rules are as follows:

Candidates who are eligible for the Mop Up Round:

• Candidates who were allotted a seat and were admitted in Round 1 and Round 2 and secured the admission but did not fill the status retention form.
• Those Candidates who have been waitlisted and whose choice was not available during Round 1 and Round 2.

Candidates who are not eligible for the Mop Up Round:

• Candidates who have been allotted a seat but haven't joined.
• Candidates who have filled the Status retention form.
• Candidates  who joined and resigned from the seats allotted.

Because you were allotted a seat and did not accept it, you now are ineligible for the Mop Up Round.